ON CHEMICAL INTERACTION OF SUBSTANCES IN THE SOLID STATE 43 



both cases the colours were quite bright after resting for 5 or 6 days, with 

 occasional shakings during this time, but even later than this some increase 

 in depth of colour was probably noticeable after still longer exposure. 



Wben the tubes were first sealed up the powders were thoroughly loose 

 and free from caking together, as could be seen by cautiously revolving each 

 tube about its axis before shaking the powders together, but they became 

 very perceptibly caked after mixture when the tubes had lain at rest for a 

 number of days, and quite vigorous shaking was then necessary to break up 

 again the caked material into loose pulverulent form and allow of further 

 mixture. This tendency to caking was seeemingly more marked in the case 

 of the mercuric than in that of the lead salt. 



In other experiments the effect of rapid and violent shaking for as 

 much as an hour was observed in the more speedy increase in depth of 

 colour, and a like effect was obtained by breaking the tube and pressing or 

 rubbing the mixed powder in a dry mortar, with inevitable exposure, of 

 course, to atmospheric moisture, the influence of which was afterwards ex- 

 aggerated by breathing upon the powder. 



In order to test further the extent to which action between the solid 

 materials might be referable to vaporization from the surface of either or 

 both of them, the experiments were repeated in longer tubes by drying as 

 already described, then introducing a parcel of phosphorous pentoxide (lying 

 apart from the other materials), sealing the tube at one end, exhausting 

 from the other with a mercurial piunp, sealing up the open end with a fairly 

 good vacuum within, and allowing the tube to lie at rest for several weeks 

 before drawing down the tube between the phosphorus pentoxide and the 

 other materials, sealing up the part of the length containing the latter. No 

 material difference of result was thus obtained. 



The general conclusion seems to be Justified that the particles of the 

 substances experimented on were, while still in the solid state, brought to 

 a considerable extent within their mutual reach so that chemical interaction 

 took place. The possibility must be admitted that even the means used 

 did not secure absohite exclusion of moisture, and the slight differences as 

 to time which were noticed between the production of lead iodide and mer- 

 curic iodide suggest the possibility that the greater volatility of mercuric 

 salts as compared with those of lead may not have been entirely without 

 influence. But in the main we seem warranted in referring the effects de- 

 scribed to chemical action between solids. 



