Heat by the Method of Mixtures. 281 



position that the trap door in the bottom of the car is just 

 over that in the water jacket. A pin fixed in proper position 

 on the track knocks away the catch on the trap door of the 

 car just as the latter comes to rest, and the body falls into the 

 basket (which is held at its highest point by the overweight on 

 the end c of the lever), precipitating the latter, both by its 

 momentum and weight, downward into the water in the calori- 

 meter. As the lever goes down the observer lets go the string 

 which holds the trap doory open, and it also closes. The car, 

 relieved of the weight of w, is pulled back out of the way by 

 the attached weight. The observer himself has then only to 

 observe temperatures while keeping the water in constant and 

 complete agitation by moving the end g of the lever up and 

 down. 



The calorimeter is readily removed when temperature obser- 

 vations have been completed, by unhooking the wires, sliding 

 the water jacket to one side from under the track, and lifting 

 off the cover of the latter. The thermometer and wire basket 

 remain in the calorimeter vessel, and are weighed with it, thus 

 avoiding any loss of water, etc., consequent upon their 

 removal. 



This apparatus, although it may seem at first sight compli- 

 cated, is in reality very simple, and readily put together by 

 any one having a small shop in his laboratory, in one or two 

 days. The great advantage resulting from its use will be 

 readily appreciated. It enables, in the first place, one observer 

 to do all the work of observation, although it is convenient to 

 have some one to write down the thermometer readings. The 

 use of the wire basket to catch the hot body, rather than allow- 

 ing it to fall directly into the calorimeter, has three advan- 

 tages : (1) it prevents splashing the water, and therefore allows 

 the calorimeter to be filled nearly full, thus obtaining a maxi- 

 mum volume with a minimum of radiating surface ; (2) it 

 allows of a more thorough agitation of the water, the body 

 itself being moved through it, and hence a more rapid equali- 

 zation of temperature in the calorimeter ; (3) it prevents any 

 danger of breaking the thermometer bulb or injuring the sides 

 or bottom of the calorimeter when a heavy body is dropped 

 into it. 



The use of the car to transfer the body from the heating 

 chamber to the calorimeter not only prevents a loss of heat 

 during the transfer, but also enables a number of small frag- 

 ments, or even a mass of powder of the substance, to be used 

 without the necessity of using an enclosing bulb, a device 

 which always renders the equalization process slower and more 

 uncertain. 



