1800.] 



Experiments on Vapour-density. 



experiment, and it is concluded, therefore, that sparking does not 

 produce any permanent dissociation, although very probably a 

 momentary dissociation is caused near the terminals when a spark 

 passes. The length of the spark in the iodine vapour was about 

 1 inch ; the coil was capable of giving a 3-inch spark in air. By 

 opening the tube under water, it was found to contain about 5 per 

 cent, of air ; this would tend to aid dissociation, and would in no way 

 lessen the value of the experiment. 



The pi-obable experimental error was about 1 per cent., and was 

 caused by the difficulty in reading the distance between the loops as 

 indicated by the arrangement of the silica. 



Vapour-density of Sulphuric Anhydride. 



The apparatus used to determine the vapour-density of sulphuric 

 anhydride was similar to that used for iodine. The only work 

 that has been done on this subject is apparently that by Schultz- 

 Sellack ( Poggendorff's Annalen,' vol. 139, 1870, p. 480) and 

 Mitscherlich (' Watts's Dictionary'), who both found the vapour- 

 density normal. Some of the trioxide was introduced into a small 

 flask, the neck of which was then sealed on to the tube from the 

 inner globe ; it was then sublimed into the globe, which had been 

 previously exhausted, by heating the flask. The vapour-density was 

 then determined in the usual way. The trioxide was absorbed by 

 water, and estimated by means of a standard ammonia solution. 

 Using cochineal as indicator, it was found possible to titrate accu- 

 rately to one drop of a decinormal solution in the following way : 

 The relative value of seminormal (approximately) solutions of acid 

 and ammonia was found, and these solutions were then diluted to 

 decinormal strength; a neutral solution was then made by using 

 decinormal acid and ammonia in the proportion found for the semi- 

 normal solutions, and this was used as a standard neutral solution, 

 with which the solutions titrated were compared. The chief difficulty 

 was the formation of sulphuric acid above the stopcock ; it was 

 partially obviated by introducing a horizontal bulb-tube above the 

 stopcock this retained most of the acid. Only one series of experi- 

 ments was made ; the first result is very nearly at saturation-pres- 

 sure. 



