272 Dr. Woods on the Heat of Chemical Combination. 



I filled a glass tube with an ounce of acidulated water (one 

 part by measure of sulphuric acid spec. grav. 1*84 to six parts 

 of water) ; I put two platina leaves into the fluid, and sending a 

 galvanic current through it by means of the twelve-cylinder bat- 

 tery, I decomposed the water. 1^ cubic inch of gas was given 

 off in the minute ; and the tangent instrument having been in- 

 cluded in the circuit, the needle rose to 21 degrees on the scale. 

 The current passed through the water for two minutes, and the 

 temperature rose from 75° to 83° F. I now removed the platina 

 leaves from the fluid, and substituted a platina wire of such a 

 length as kept the needle, the current having passed through the 

 wire, at 21 degrees. The same fluid into which the electrometer 

 leaves had been immersed now surrounded the wire; the current 

 in the former case passing through the water and consequently 

 tvith decomposition, in the latter through the wire without it ; 

 and after traversing the wire for the same length of time, the 

 temperatm-e of the fluid was raised from 75° to 90°, that is 15 

 degrees, or 7° more than in the case where decomposition accom- 

 panied it ; although in both cases the same resistance was ofi^ercd 

 to the curi'ent, and of course the same amount of heat generated ; 

 therefore the 7° were absorbed by the decomposition. In the two 

 minutes that the experiment lasted, 3 cubic inches of gas were 

 given off : now if these gases were made to combine chemically, 

 what amount of heat would they produce ? The same that their 

 separation caused a loss of, viz. about 7° F. to an ounce of water 

 acidulated as above ; for 3 cubic inches of the mixed gases in 

 proper proportion to form water weigh '387 gr. ; and as oxygen 

 is f ths of the mixtui'e, it must weigh '344 gr. Now as 1 lb. of 

 oxygen combining with hydrogen would cause the temperature 

 of 43 lbs. of water to rise 180°, 1 grain would raise the tempe- 

 ratui-e of 43 grs. 180° F. ; then '344 gr. would raise the tempe- 

 rature of 43 grs. 62° F., or of 1 oz. very nearly 5°-6 F. But the 

 specific heat of the acidulated water to that of distilled water is 

 as 8'3 to 10 ; therefore the 5°*6 must be raised inversely in that 

 proportion, or to 6°*7. In our experiment very nearly the same 

 amount was lost by their separation. j^ p 



(11.) I tried the experiment in another way. Through 

 the bottom of a glass tube I passed two short pieces of thick 

 platina wire, and attached to each piece a fine wire which 

 reached to the top of the vessel, as at AB. * I could join 

 the wii'es at the bottom so as to make the two thin wires 



into one, or separate them by disuniting the thick pieces outside 

 the bottom of the tube. In the former case the current passed 

 through the wire, in the latter through the water. Each offered 

 the same degree of opposition to the passage of the cun-ent. 

 Here the same wires that acted as the leaves of the electrometer 



