2 PROCEEDINGS OF THE AMERICAN ACADEMY 



already well investigated, either in regard to their viscosity or other 

 properties, making measurements of the viscosity at temperature in- 

 tervals of about 20° from low to higli temperatures. But the laborious 

 nature of the observations, and the large amount of time demanded by 

 the observations and their reduction, have rendered the fulfilment of 

 the plan impossible for one whose available time has been fully occupied 

 with laboratory instruction. It will be seen that the last of the obser- 

 vations given were made iu April, 1880. Since that date it has been 

 wholly inconsistent with the author's duties and health to continue the 

 work, and the incomplete results are now presented in the hope that, 

 despite their limited range, they may be a contribution of some perma- 

 nent value to the subject treated. 



The expenses of the investigation have been mainly defrayed by an 

 appropriation granted by the American Academy of Arts and Sciences 

 from the Rumford Fund for researches on Light and Heat. 



The objects of the investigation were concisely stated in the paper 

 of 1876. The extension of the work was looked to for the develop- 

 ment of data which not only should furnish another experimental check 

 on the deductions of the kinetic theory of gases, but should provide 

 material of sufficient precision (approximating to that with which the 

 coefficient of expansion of gases is known) to serve as a part of that 

 experimental basis on which the complete theory of gases, and indeed 

 of molecular physics in general, must eventually rest. The critical re- 

 view of the method, given at the end of the paper, is intended to furnish 

 the material for the formation of a judgment as to how far this object 

 has been fulfilled in the case of the gases investigated. 



Apparatus. 



The apparatus used is shown in the schematic section given by the 

 accompanying cut. It is in principle the same as that used in the for- 

 mer measurements. The dried gas is trauspired successively througli 

 two glass capillaries of about 30 cm. in length, from a space containing 

 the gas at a measured pressure to another space from which the gas is 

 continuously exhausted as fiist as it enters, and which is thus maintained 

 at a constant and measured pressure. The pressure of the gas in the 

 intermediate space is also measured. The capillaries are maintained at 

 known or measured constant temperatures, AB being usually packed 

 in finely pounded ice, and DC being in a jacketed double oil-bath, of 

 which the temperature is maintained nearly constant by regulating the 

 flame of the gas-burners, and is measured by the thermometer with its 



