Graham found that the passage of gases through minute openings could be much more accurately studied by placing the gas to be examined in a glass tube one end of which was closed by a plug of dry plaster of Paris, than by using vessels with small fissures in the glass.
The diffusion-tube used by Graham generally consisted of a piece of glass tubing, graduated in fractions of a cubic inch and having a bulb blown near one end; the short end was closed by a thin plug of dry plaster of Paris (gypsum), the tube was filled with the gas to be examined, and the open end was immediately immersed in water. The water was allowed to rise until it had attained a constant level, when it was found that the whole of the gas originally in the tube had passed outwards through the porous plug, and air had passed inwards. The volume of gas originally in the tube being known, and the volume of air in the tube at the close of the experiment being measured, it was only necessary to divide the former by the latter number in order to obtain the number of volumes of gas which had passed outwards for each one volume of air which had passed inwards; in other words to obtain the rate of diffusion compared with air of the gas under examination.
Graham's results were gathered together in the statement, "The diffusion-rates of any two gases are inversely as the square roots of their densities." Thus, take oxygen and hydrogen: oxygen is sixteen times heavier than hydrogen, therefore hydrogen diffuses four times more rapidly than oxygen. Take hydrogen and air: the specific gravity of hydrogen is 0·0694, air being 1; the square root of 0·0694 is 0·2635, therefore hydrogen will diffuse more rapidly than air in the ratio of 0·2635:1.
In the years 1846-1849 Graham resumed this inquiry; he now distinguished between diffusion, or the passage of gases through porous plates, and transpiration, or the passage of gases through capillary tubes. He showed that if a sufficiently large capillary tube be employed the rate of transpiration of a gas becomes constant, but that it is altogether different from the rate of diffusion of the same gas. He established the fact that there is a connection of some kind between the transpiration-rates and the chemical composition of gases, and in doing this he opened up a field of inquiry by cultivating which many important results have been gained within the last few years, and which is surely destined to yield more valuable fruit in the future.
Returning to the diffusion of gases, Graham, after nearly thirty years' more or less constant labour, begins to speculate a little on the causes of the phenomena he had so studiously and perseveringly been examining. In his paper on "The Molecular Mobility of Gases," read to the Royal Society in 1863, after describing a new diffusion-tube wherein thin plates of artificial graphite were used in place of plaster of Paris, Graham says, "The pores of artificial graphite appear to be really so minute that a gas in mass cannot penetrate the plate at all. It seems that molecules only can pass; and they may be supposed to pass wholly unimpeded by friction, for the smallest pores that can be imagined to exist in the graphite must be tunnels in magnitude to the ultimate atom of a gaseous body." He then shortly describes the molecular theory of matter, and shows how this theory—a sketch of which so far as it concerns us in this book has been given on pp. 123-125—explains the results which he has obtained. When a gas passed through a porous plate into a vacuum, or when one gas passed in one direction and another in the opposite direction through the same plate, Graham saw the molecules of each gas rushing through the "tunnels" of graphite or stucco. The average rate at which the molecules of a gas rushed along was the diffusion-rate of that gas. The lighter the gas the more rapid was the motion of its molecules. If a mixture of two gases, one much lighter than the other, were allowed to flow through a porous plate, the lighter gas would pass so much more quickly than the heavier gas that a partial separation of the two might probably be effected. Graham accomplished such a separation of oxygen and hydrogen, and of oxygen and nitrogen; and he described a simple instrument whereby this process of atmolysis, as he called it, might be effected.
Graham's tube atmolyser consisted of a long tobacco-pipe stem placed inside a rather shorter and considerably wider tube of glass; the pipe stem was fixed by passing through two corks, one at each end of the glass tube; through one of these corks there also passed a short piece of glass tubing. When the instrument was employed, the piece of short glass tubing was connected with an air-pump, and one end of the pipe stem with the gaseous mixture—say ordinary air. The air-pump being set in motion, the gaseous mixture was allowed to flow slowly through the pipe stem; the lighter ingredient of the mixture passed outwards through the pipe stem into the wide glass tube more rapidly than the heavier ingredient, and was swept away to the air-pump; the heavier ingredient could be collected, mixed with only a small quantity of the lighter, at the other end of the pipe stem. As Graham most graphically expressed it, "The stream of gas diminishes as it proceeds, like a river flowing over a pervious bed."
Graham then contrived a very simple experiment whereby he was able to measure the rate of motion of the molecules of carbonic acid. He introduced a little carbonic acid into the lower part of a tall cylindrical jar, and at the close of certain fixed periods of time he determined the amount of carbonic acid which had diffused upwards through the air into the uppermost layer of the jar. Knowing the height of the jar, he now knew the distance through which a small portion of carbonic acid passed in a stated time, and regarding this small portion as consisting of a great many molecules, all moving at about equal rates, he had determined the average velocity of the molecules of carbonic acid. A similar experiment was performed with hydrogen. The general results were that the molecules of carbonic acid move about in still air with a velocity equal to seventy-three millimetres per minute, and that under the same conditions the molecules of hydrogen move with a velocity equal to about one-third of a metre per minute.[12]
The Bakerian Lecture for 1849, read by Graham before the Royal Society, was entitled "On the Diffusion of Liquids." In this paper he describes a very large number of experiments made with a view to determine the rate at which a salt in aqueous solution diffuses, or passes upwards into a layer of pure water above it, the salt solution and the water not being separated by any intervening medium. Graham's method of procedure consisted in completely filling a small bottle with a salt solution of known strength, placing this bottle in a larger graduated vessel, and carefully filling the latter with water. Measured portions of the water in the larger vessel were withdrawn at stated intervals, and the quantity of salt in each portion was determined. Graham found that under these conditions salts diffused with very varying velocities. Groups of salts showed equal rates of diffusion. There appeared to be no definite connection between the molecular weights of the salts and their diffusion-rates; but as Graham constantly regarded diffusion, whether of gases or liquids, as essentially due to the movements of minute particles, he thought that the particles which moved about as wholes during diffusion probably consisted of groups of what might be called chemical molecules—in other words, Graham recognized various orders of small particles. As the atom was supposed to have a simpler structure than the molecule (if indeed it had a structure at all), so there probably existed groups of molecules which, under certain conditions, behaved as individual particles with definite properties.
As Graham applied the diffusion of gases to the separation of two gases of unequal densities, so he applied the diffusion of liquids to the separation of various salts in solution. He showed also that some complex salts, such as the alums, were partially separated into their constituents during the process of diffusion.
The prosecution of these researches led to most important results, which were gathered together in a paper on "Liquid Diffusion applied to Analysis," read to the Royal Society in 1861.