Prof. J. Joly on the 



The chief interest of these results is the support which 

 they give to the view that there exist variations in stability 

 among these compounds which increase per saltum and with 

 a certain regularity as measured by the temperature required 

 for decomposition. If this can be sustained a useful method 

 of discriminating in some cases between true and erroneous 

 structural formulae is to be found in such observations ; and, 

 in addition, we seem to obtain information as to the relative 

 stabilities of various interatomic relations. I shall point out 

 some features of the results bearing upon the construction I 

 have placed upon them. 



The first question that arises is, of course, how far the 

 suggested subdivisions are actually justified by the observa- 

 tions. In other words, how many and what exceptions and 

 objections to the grouping can be detected? The grouping 

 assumes that a compound begins to sublime at the particular 

 approximate temperature of its group and decomposes at 

 or about this temperature. Now it is often found that a 

 considerable increase of temperature is required if the subli- 

 mation or decomposition of the specimen is to be maintained. 

 If the increase is not more than 50 or 100 degrees we may 

 explain this very simply as due to the necessity for heating 

 the upper particles on the hob as well as breaking up the 

 larger grains of the powder. Sometimes, however, an eleva- 

 tion of temperature is required which carries the temperature 

 into the succeeding group. In nearly all cases when this has 

 occurred, if not in all, there is a distinct interval of tempera- 

 ture over which no sublimation is obtained. This interval 

 may be 100 or 200 degrees, and in this interval no trace of 

 sublimate appears, at least during reasonably prolonged 

 periods of observation. When sublimation recommences, 

 the lower limit of the higher group has been entered and 

 sublimation proceeds just as if the substance had commenced 

 its sublimation in this group. In the new group we must 

 again raise the temperature to maintain the sublimation. A 

 deflagration point may be reached, attended, in some cases, 

 by a copious evolution of sublimate, and after this a rapid 

 decline in the rate of expulsion of volatile matter. There 

 may succeed yet another interval of temperature in which 

 no sublimation occurs, and at a temperature which may lie 

 above that defining the highest group — at 800° or higher — a 

 final evolution of sublimate may take place. On account 

 of the occasional occurrence of this phenomenon I have 

 numbered the groups from below upwards, as I think it 

 probable that further observation may show the existence of 

 a higher group. 



