EXPERIMENTAL KNOWLEDGE OF THE PROPERTIES OF MATTER. 139 



ture of boiling sulphur 440°, boiling cadmium ^ [860°], and boiling zinc 

 [1040°]. In the first and last cases it was used as the thermometric 

 substance, i.e. the expansion of the iodine (assumed as obeying Gay- 

 Lussac's law) was known by the amount left in the flask after the 

 experiment was over ; and as the vapour of iodine is heavy, iodine should 

 be an accurate thermometric substance, used in this way by weighing the 

 iodine left in the flask. Unfortunately it has since been found that iodine 

 does not obey Gay-Lussac's law above 590°, above this temperature its 

 rate of expansion increasing. 



But in Deville and Troost's experiments the (relative) vapour-density 

 of iodine is almost the same at [860°] as at 440°, viz., 87.^ This seems 

 inconsistent with what was said just now ; there may be some error here, 

 or it may be, as is likely from analogy, that the dissociation of iodine 

 molecules imagined by V. Meyer, and by Crafts and Meier, may be a slow 

 process requiring more time than was given in this experiment. 



However that may be, the use of iodine as a thermometric substance 

 for giving the boiling-point of zinc was not legitimate, and Deville and 

 Troost themselves have since found that^ the boiling-point of zinc was 

 over-estimated by 100°, the true boiling-point of zinc being in fact 940° ; 

 the more than normal expansion of iodine at 940° had given a result due 

 to a normal expansion at 1040°. 



The cubic expansion of the porcelain of which the flasks were made 

 was determined by Deville and Troost and found to amount to "009288 

 between 0° aaid the boiling-point of cadmium, Avhich was supposed to be 

 860° but is now known to be about 772°, as found by Carnelley and 

 Carleton Williams (' J.C.S.' 1878, xxxiii. 284.) 



Third Report of the Committee, consisting of Professor Balfour 

 Stewart {Secretary), Mr. J. Knox Laughton, Mr. G. J. Symons, 

 Mr. R. H. Scott, and Mr. Johnstone Stonet, appointed for the 

 purpose of co-operating with Mr. E. J. Lowe in his project of 

 establishing a Meteorological Observatory near Chepstotv on a 

 permanent and scientific basis. 



In their last report this Committee, after expressing their opinion 

 that the establishment of a permanently endowed meteorological observa- 

 tory on a good site, such as that of Shire Newton, is a matter of un- 

 deniable scientific importance, instructed their Secretary to write as 

 follows to Mr. Lowe : — 



' The Committee request me to point out to you that the main feature 

 of your proposal, which interests the British Association and the scientific 

 public generally, is the prospect which it holds out of the establishment 

 of a permanent institution by means of which meteorological constants 

 could be determined, and any secular change which may take place 

 therein in the course of a long period of years be ascertained. It will be 

 for you and the local authorities to decide what a,mount of work of local 

 interest should be contemplated, and on this will the scale of the observa- 



• C.R. xlix. p. 240. - Ann. Chim. et Phys. 1S60, Iviii. p. 285. 



3 C.E. xc. p. 793. 



