142 APPENDIX II. 



hippuric acids are the only ones in appreciable 

 quantity which still retain potential energy on 

 leaving the body, and of these the two latter are 

 excreted in such small proportions that they may be 

 considered as urea without introducing any material 

 error into the results. 



These determinations were made in Lewis Thomp- 

 son's calorimeter, ^vhich consists of a copper tube 

 to contain a mixture of chlorate of potash with the 

 combustible substance, and which can be enclosed in 

 a kind of diving-bell, also of copper, and so lowered 

 to the bottom of a suitable vessel containing a 

 known quantity (2 litres) of water. The deter- 

 minations were made with this instrument in the 

 following manner : 19' 5 grams of chlorate of pot- 

 ash, to which about -|th of peroxide of manganese 

 was added, was intimately mixed with a known 

 weight (generally about 2 grams) of the substance 

 whose potential energy was to be determined, and 

 the mixture being then placed in the copper tube 

 above mentioned, a small piece of cotton thread 

 previously steeped in chlorate of potash and dried 

 was inserted in the mixture. The temperature of 

 the water in the calorimeter was now carefully 

 ascertained by a delicate thermometer ; and the end 

 of the cotton thread being ignited, the tube with its 

 contents was placed in the copper bell and lowered 

 to the bottom of the water. As soon as the com- 

 bustion reached the mixture a stream of gases issued 

 from numerous small openings at the lower edge of 

 the bell and rose to the surface of the water a 

 height of about 1.0 inches. 



