Temperature of Sublimation. 857 



Since the writing of my first paper I have been able to 

 observe the temperatures of sublimation of a certain number 

 of typical minerals, and although more observations are 

 desirable, the approximate agreement among the results 

 obtained, as well as the delay which must attend the 

 acquisition of fresh material, induce me to give a brief 

 account of what I have observed. I shall in the present 

 paper refer to four volatile constituents only — antimony, 

 arsenic, sulphur, and tellurium. 



The temperature of sublimation is not a fixed and definite 

 point. Substances such as arsenic, sulphur, &c, are more 

 or less volatile even at ordinary temperatures, and the rate 

 of volatilization increases as the temperature rises. When 

 the free element is being dealt with in presence of oxygen, 

 we have to do with this range of increasing vapour-pressure 

 as well as with the complication arising from the interaction 

 of the element with oxygen. In this case the temperature 

 of sublimation is not in general that of the oxide formed, but 

 seems dependent mainly on the vapour-pressure of the volatile 

 element at the particular temperature. In the case of com- 

 pounds the stability of the compound in presence of oxygen 

 must be the primary factor controlling sublimation at a rising 

 temperature. Bow far the vapour-pressure of the volatile 

 element in the compound affects the temperature of decom- 

 position cannot be predicted, but, save in the case of deflagra- 

 tion or similar phenomena, the decomposition temperature 

 does not present itself in the experiments as a definite and 

 sharply defined one, and the observation resolves itself into 

 finding the temperature at which the liberated volatilized 

 body, whether oxidized or unoxidized, first appears as a 

 visible sublimate on the watch-glasses ; most often on the 

 upper glass. We are here presented with much the same 

 problem as that of determining the melting-point of a body 

 having a certain range of viscosity. A time element enters 

 into the observation. Accordingly I have allowed a fixed 

 time — three minutes — in each case for the sublimate to 

 collect. At the end of that time the hob is cooled and the 

 upper glass removed and examined in a good light. The 

 sublimate as first revealed may be a mere bloom or dullness 

 upon the glass. With a longer allowance of time a some- 

 what lower temperature would be obtained. The tempera- 

 tures recorded are, therefore, approximate only, and actually 

 indicate a certain rate of break up of the substance under 

 examination. But they are comparable one with another; 

 and in so far are indicative of the relative stabilities of the 



