PREFACE. 



Observing that the Cartesian diver used in my lectures since 1895 grew 

 heavier from year to year, I resolved in 1900 to make definite measurements 

 of the rate of loss of buoyancy, believing that these would be fruitful ; they 

 would bear directly on the coefficient of diffusion of the imprisoned gas 

 through the liquid in which the diver is floating; it would be easily possible 

 to vary the liquids and gases, within and without, under conditions of a 

 determinable diffusion gradient. Ultimately the transfer of single mole- 

 cules of a gas through the intermolecular pores of the liquid is in question, 

 so that the experiment might throw definite light on the size of physical 

 pores and on the other molecular relations involved. 



The experiments in Chapter I, made during a period of eleven years, with 

 an ordinary glass balloon-shaped Cartesian diver with a small aperture, 

 culminated in a plausible value of the diffusion coefficient {i. e., grams of 

 gas or standard volume of gas transpiring per second across an orthogonal 

 square centimeter, in case of a unit pressure gradient) of the imprisoned air 

 through water, together with suggestive relations of the mean viscosity of 

 the imaginary medium within the molecular pores of the liquid through 

 which a single molecule of the gas virtually transpires. The investigation 

 was therefore taken up on a more extended scale, for different pairs of gases. 



In Chapter II the diver is modified in form and the endeavor is made to 

 obtain equal areas in the section of the cylindrical swimmer and the annular 

 space without, in order to conform more closely to the equation of diffusion. 

 The theory of the phenomenon and the errors involved are discussed. It 

 appears that, even for mixed gases, the volumes diffusing (if not the masses) 

 are fully determinable. The accuracy essentially depends on the measure- 

 ment of absolute temperature and of barometric pressure and should there- 

 fore be of an order below 1/2730 per o.i° C. or 1/7600 per 0.1 centimeter of 

 mercury. As the masses of gas contained are as a rule much less than io*~ 2 

 gram, even in case of air, the weight less than 0.000004 gram is determinable, 

 showing the remarkable sensitiveness of the method . Moreover, in the region 

 of constant temperature, the limit of sensitiveness is immensely greater. 



In order to elucidate the phenomenon, experiments were begun with the 

 transpiration of imprisoned hydrogen into air, in which the resultant diffu- 

 sion is always unidirectional, outward from the diver. Initially rates as 

 large as 5 mg. per day were obtained, which eventually decreased to a con- 

 stant value, equivalent to a fixed diffusion coefficient which indicated the 

 diffusion of air only. The case of air into air through water showed a 

 definite mean rate throughout the two or three months of observation ; but 

 the daily march of the loss by diffusion was remarkably irregular, a result 

 finally referred to the change of solubility of the gases in water with tern- 



