W. G. Mixter — Heat of Oxidation of Tin. 231 



was a mass of elongated microscopic crystals. Professor W. 

 E. Ford of the Mineralogical Laboratory has kindly examined 

 the crystals and found them too small for measurement but 

 having the optical properties of cassiterite. It should be 

 stated that a small portion of the stannic oxide was deposited 

 on the upper surface of the bomb in the form of white 

 powder. After a combustion the oxide was transferred to a 

 platinum dish and the water used in washing out the last 

 portions was evaporated. The weight of the oxygen which 

 combined the tin was found as already described. The follow- 

 ing are the experiments : 



3 4 



Tin ._ 15*168 grams 14*055 grams 



" equivalent to oxygen com- 

 bined „__ 15-042 " 13-919 " 



Oxygen combined 4*045 " 3'743 " 



Water equivalent of system 3548- " 3658* " 



Temperature interval". 4-893° " 4-410° " 



Heat observed 17360 C 16132 c 



" of oxidation of iron . 41 c 40 c 



17319 C 16092 



For 1 gram of oxygen 4282 c 4293 c 



The average of the results is 4288 c for one gram of oxygen 

 combining with tin to form stannic oxide. For 32 grams it is 

 137200 c at constant volume and 137800 c at constant pressure. 

 It will be observed that the tin in the form of foil was almost 

 completely oxidized. Andrews' result was 136000° at constant 

 pressure. In his experiments there may have been formed 

 some amorphous stannic oxide which would lower the heat 

 effect. His result, however, agrees well with the writer's. 



Stannous Oxide. 



A number of preparations of stannous oxide were made by 

 different methods but only two appeared to be good enough for 

 the purpose. The first used was made by adding an excess of 

 ammonia to a solution of stannous chloride and then heating the 

 mixture several days on a steam bath. The dark crystalline mass 

 was washed thoroughly and dried at 100° and then heated in a 

 current of dry carbon dioxide to about 400° as long as water 

 and ammonia came off. The product was almost black, showing 

 under the microscope minute crystals but no amorphous powder. 

 It contained considerable stannic oxide, a trace of ammonia, and 

 0*09 per cent of water. Three determinations of the heat of 

 oxidations gave respectively : 4668 c , 4667 c and 4632 c , a mean of 



