208. PROCEEDINGS OP THE AMERICAN ACADEMY 



inside a second cylindrical vessel of the same material, leaving an an- 

 nular space about two inches wide and an equal space between the 

 bottoms of the two vessels, all of which is filled with water. In addi- 

 tion, the water vessel is further protected by a layer of felt one inch 

 thick kept in place by an outer covering of the same sheet metal. 

 When in use, the top is covered by a cap made also of thick felting, 

 and in the crown of the cap is a slit, through which a thermometer may 

 be inserted or the hand passed to turn the stop-cocks. The gas to be 

 used is conducted into the calorimeter through a very small lead tube, 

 which is connected with the lower tubulature of the stop-cock by a 

 rubber connection, and the balloon is so placed that the current shall 

 enter either at the top or at the bottom of the vessel, according as the 

 gas is lighter or heavier than the atmosphere. The calorimeter case 

 stands about four feet high, and the balloon rests in it on a down cush- 

 ion, with the connecting tube's fully six inches below the surface of the 

 water in the annular space above mentioned. "With this apparatus it 

 was possible to keep the temperature of the balloon constant within 

 the one hundredth of a centigrade degree for an hour at a time, unless 

 the temperature of the laboratory suddenly and greatly changed. 



The general arrangement for filling the balloon was then as follows. 

 The gas delivered from the drying tubes of one of the generators here- 

 after to be described passed through the lead tube of which we have 

 spoken to one or the other, as the case might be, of the tubulatures of 

 the balloon standing in the calorimeter. Passing out by the opposite 

 tubulature, the current was conducted by a similar lead tube to a 

 second but much larger balloon serving as a gasometer, and from the 

 last vessel through a gas wash-bottle to the atmosphere. The bottom 

 of the wash-bottle was covered with strong sulphuric acid to dry any 

 returning air, but not deep enough for the inlet tube to dip under the 

 liquid. The current was continued until the proper test showed that 

 the gas issuing from the end of the apparatus was perfectly pure. 

 Then the inlet cock of the balloon was turned, and the temperature of 

 the calorimeter case watched until constant, and a sufficient time had 

 elapsed to establish a perfect equilibrium with the atmosphere through 

 the connecting tubes and vessels. This, as we found by experience, 

 often requires longer time than we were led to anticipate, and when 

 experimenting with carbonic acid gas it is not safe to allow less than 

 twenty or thirty minutes ; and now the large balloon and wash-bottle 

 come into play to prevent air diffusing into the balloon, or being 

 drawn back in consequence of changing pressure. 



When all the conditions seemed to be satisfactory, the temperature 



