130 



exclusively from these expressions and the ratio of the specific heats 

 as determined by the theory of sound, are given. These tables 

 show the mechanical values of the specific heats of air at different 

 constant pressures, and at different constant densities. Taking 

 1390 as the mechanical equivalent of the thermal unit as determined 

 by Mr. Joule's experiment on the friction of fluids, the authors 

 find, as the mean specific heat of air under constant pressure, 



2390, from to 100 Cent. 



2384, from to 300 Cent. 



XII. " Note on Nitro-glycerine." ByA.W.WiLLiAMSON,Ph.D., 

 F.C.S.j Professor of Practical Chemistry in University Col- 

 lege. Communicated by Dr. SHARPEY, Sec. R.S. Received 

 June 15, 1854. 



This compound is formed by acting upon glycerine with a mix- 

 ture, in equal volumes, of concentrated nitric and sulphuric acids, 

 the glycerine being added by a few drops at a time. 



It is heavier than water, in which it is slightly soluble, and is 

 soluble in alcohol and in ether. 



From its proneness to decomposition in drying, even by the air- 

 pump, a complete analysis could not be made, but a qualitative ex- 

 amination of the relative amounts of carbon and nitrogen gave the 



following results : 



1. 2. 3. 4. 



Volumes of mixed gases 101 91*5 99 97 



Volumes of nitrogen not absorbed by potash. . 32 30'5 34 33 



Carbonic acid absorbed by potash 69 61 65 64 



1. 2. 3. 4. 5. 



Mixed gases 178 194 173 194 192 



Nitrogen 61 66 58 65 65 



CO 2 117 128 115 129 127 



From these results the following formula was deduced : 



C 6 H 8 6 + 3N0 5 =C 6 3 *J^ 6 + 3HO. 



It would therefore appear that 3H are replaced by 3NO 4 . 

 On boiling this compound with concentrated solution of potash, it 

 is decomposed into glycerine and nitrate of potash. 



