SALT SOLUTIONS AND THE ABSORPTION OF GASES. By E. L. NICHOLS, of Baltimore, Md.

THE experiments described in a paper presented by Mr. Wheeler and myself, on the Coefficient of Expansion of Gas-solutions, give new evidence of an analogy, which I had for some time suspected to exist between the processes of salt-solution and the absorption of gases by liquids. The phenomena, by reference to which this analogy can be best shown, are too familiar to need description. It is, however, necessary to consider the explanation of them, afforded by the kinetic gas theory; it being by means of this theory that the relations in question can be most simply expressed.

When a liquid and a confined portion of gas have been for some time in contact, a constant condition is reached. In common parlance, the liquid is then saturated with the gas, and the vapor in the atmosphere above the liquid is at its maximum tension for that temperature and pressure. Now, in the terms of the kinetie theory of gases, the state of things is as follows: There are two fluid mixtures having one surface in common, across which surface a double interchange of molecules is continually taking place. The upper mixture consists of two kinds of molecules moving through long, free paths; the lower of two kinds of molecules (chemically identical with the first two varieties) moving through short free paths. The rate at which the molecules cross the common surface differs according to their kind, but the number of each kind crossing in one direction, in a unit of time, is precisely equal to the number of the same kind crossing in the opposite direction. Upon this last fact depends the maintenance of the constant condition in the two halves, and it is to this fact we refer when we speak of the saturation of the liquid, and of the maximum tension of the vapor.

Suppose, on the other hand, that a liquid and some solid which is soluble in it are in contact. After a time, as in the former case, a constant state is reached, which state we also express by the term saturation. We are not able to tell with the same certainty, what takes place between solid and liquid, because the kinetic gas theory has not been extended to cover these cases; but from the theory as at present understood, we may safely

assume that the liquid, after saturation, consists of an aggregation of independently moving molecules, making very short excursions. It is probably composed of two sets of molecules; molecules of the original liquid, and molecules of the dissolved salt. By molecules are meant, simply, such particles as in a gas or liquid move independently. I shall presently give reasons for assuming the solution to be a mixture of this kind rather than a collection of complex molecules, such as might be formed by any combination of the salt and liquid molecules.

To account for saturation, we must assume one of two things: that the rate at which particles leave the solid mass, during the process of solution and begin a course of independent excursions throughout the solution, grows slower from time to time, and that this process finally ceases entirely, or that a reverse process sets in which, after a time, counterbalances the first movement. If we adopt the hypothesis that there is a return of salt-molecules to the solid, the rate of which return grows more and more rapid as the solution becomes stronger, until finally the number of particles returning in a unit of time precisely equals the number torn loose by the action of the liquid, we obtain an explanation of the saturation of salt-solutions similar to the generally accepted explanation of the saturation of liquids by gases. The process of solution can then be treated as a special case of the general process known as fluid diffusion.

The other hypothesis leads to no such simple explanation. To account for the cessation of the process of solution, it becomes necessary to reject the supposition just made concerning the constitution of the solution, and to suppose some combination of the salt and liquid into complex molecules incapable of further action upon the solid salt. The point of saturation can be thus accounted for, but many secondary phenomena, which follow directly under the first hypothesis, remain unexplained.

There are other reasons for regarding salt-solution as a process of fluid diffusion. When a body passes from the liquid into the gaseous or solid states, it experiences under certain conditions what Prof. James Thomson has called a "difficulty in beginning the change of state." It is supposed that the same difficulty would exist, when vapor is cooled to the condensing point, if it were feasible to fulfil the necessary condition. This condition is the entire absence of the substance in question in the state into which

the rest of the substance is ready to pass. Thus in the cooling of water below the freezing point, a bit of ice causes the immediate congelation of the entire mass; in the superheating of water, the presence of the smallest globule of saturated vapor below the surface of the liquid, brings about explosive boiling. Knowing this, it is easy to surmise the nature of this difficulty of beginning the change of state; the change of state is a process of fluid diffusion involving the presence of the substance in both states, and the reluctance to begin the change occurs whenever the substance is present in one state only. In the super-saturation of salt-solutions this same difficulty of beginning the change of state occurs; and the difficulty disappears when a particle of the substance in the solid state is introduced into the solution; just as in the other cases it vanished so soon as the substance was present in the two states. This indicates that the process is the same in the two cases; in a word, that, as we had concluded from other considerations, solution is a process of fluid diffusion.

The other phenomena connected with the change of state admit of equally simple explanations, if we adopt one more hypothesis concerning the nature of liquids. I feel the more confidence in making use of this hypothesis, since Professor Clausius has suggested and defended it in a recent paper in Poggendorff's Annalen. According to this view, the liquid molecule (meaning again by molecule that portion of the liquid which moves independently) is an aggregation of the gas-molecules of the substance; and its mass is a function of the temperature-decreasing as the liquid is heated. Andrews' critical point is then for each liquid, the temperature at which the liquid and vapor molecule are identical. At lower temperatures, the ability of liquid and vapor to exist, side by side, exhibiting such different properties, is explained by the difference in the size of their molecules. Under this hypothesis, the total energy necessary to the vaporization of a liquid is divided into two portions: that involved in raising the mean velocity of the molecules, and that consumed in breaking up the larger molecules into smaller ones. This latter portion is in common parlance, the latent heat of vaporization; it decreases with rise of temperature, and at the critical point becomes equal to

zero.

Under this hypothesis, finally, we can account for the high rate of evaporation of gases in solution and the low rate of salts in so

lution. During evaporation it is chiefly the molecules of highest velocity which escape - hence the lowering of the temperature of the liquid from the evaporation. If the liquid molecules of a gas solution are of much greater mass and of much smaller mean velocity than the gas molecules, the absorption of the latter will raise the mean velocity of the mixture, and cause the well-known rise of temperature duc to absorption, but the small gas particles will retain their own high velocity instead of imparting them to the large liquid molecules.

Were gas molecules and liquid molecules of nearly equal mass, a distribution of velocities would occur, since by a single collision the entire velocity of a gas molecule could be imparted to some liquid molecule; but it is evident that since no gas molecule impinging upon a very much larger liquid molecule can sensibly change the velocity of the latter, and since in the long run the sum of the velocities imparted to such a liquid molecule by such collisions is equal to zero; there can no such distribution of velocities take place. The gas molecules in the mixture will therefore retain their high velocity and will evaporate more rapidly than the liquid molecules. In the same way, if the molecules of a salt in solution are vastly larger than the liquid molecules the liquid molecules will maintain their higher velocities and the liquid component of the solution will evaporate most rapidly.

This fact, that in salt and gas solutions the different components evaporate separately and independently and at very different rates, implies their existence in the solution in independently moving molecules. The hypothesis of a complex molecule offers no such simple explanation of the phenomena connected with solution; therefore is not tenable. From the other hypothesis the classification of the process of solution together with the other processes by which matter changes state, under the head of fluid diffusion, follows almost as a matter of course.

NOTES ON WATER ANALYSIS. By A. A. BRENEMAN, of Ithaca, N. Y.

1. Significance of the chlorine test.-Notwithstanding all that has been said upon the subject, it seems to me worthy of remark, that the determination of chlorine in well waters is of the greatest importance, under certain precautions, and in many cases should alone condemn the water. My own experience in more than two hundred determinations of chlorine, in the waters of the gravel district of Central New York, determinations made together with the ordinary analysis by Wanklyn's method, sustains this view.

2. Change of Organic Matter by Aeration and Light.-A sample of peaty water, deep brownish in color, and yielding 0.44 per million of albuminoid ammonia, gave after flowing one and a half miles, with slight fall, .25, and at the end of three miles .11, the color passing from deep brown to almost nothing, at the same time.

The acration of suspended matter in streams should in general lead to a more liberal interpretation of its influence, than in the case of the same kind of matter in well waters, which are deprived of aeration. Too little importance is attached to influence of animal life and strong light as purifying agents, in running streams.

3. On the Volume of Water to be used in Water Analysis. — The use of 100 cc. for each distillation by Wanklyn's method is preferable, because of the greater convenience and quickness of the operation, and also because of increased delicacy in Nesslerizing. Increased delicacy results from the use of smaller tubes, which give a length of column greater in proportion to volume of distillate used in Nesslerizing.

4. An Improved Nessler pipette-stirrer.—The pipette used for the Nessler test, as here shown, is distinguished from the ordinary bulb-stirrer by the very short tip below the cord or bead, which latter fits closely to the tube in which the test is made. The wide opening prevents spirting of liquid from the tube as the stirrer is raised and lowered rapidly. The short tip permits agitation of the lowest portion of liquid in the tube.

5. On the use of Common Yellow Glass as a Color Standard in Nesslerizing.-The advantage of colored glass is its permanence and convenience of application. The glass in ordinary use is