ON TEACHING CHEMISTKY. 89 



The Committee think that some suggestions might now be made as to 

 the method of teaching chemistry which should be followed in schools. If 

 this can be done, it will certainly confer a great benefit on both teachers 

 and examiners, and will be likely to lead to a more emphatic recognition of 

 "the merits of the science as an instrument of elementary education. The 

 ■Committee accordingly ask for reappointment. 



Report of the Committee, consisting of Dr. Russell, Captain Abney, 

 Professor Hartley, and Dr. A. Richardson (Secretary), appointed 

 for the investigation of the action of Light on the Hydracids 

 of Halogens in presence of Oxygen. {Drawn up by Dr. A. 

 Richardson.) 



During the past year this Committee has made numerous experiments 

 on the decomposition of gaseous hydrochloric acid, under the combined 

 inflnence of sunligrht and oxygen. 



A series of bnlbs containing a mixture of moist hydrochloric acid and 

 varying quantities of moist oxygen were exposed to light for five months 

 (from December 9 to May 26) ; the amount of free and combined chlorine 

 was then determined (the details are given in Table I.). It will be seen 

 that in bulbs 1, 2 the percentage of free chlorine is only 3'6 to 3"4, rising 

 suddenly, however, in bulb 3 to 92" 5 per cent. : in No. 4 the amount of 

 chlorine liberated reaches the maximum, viz., 92'77 per cent. ; when more 

 oxygen is added the percentage of liberated chlorine is lowered, the effect 

 Ijeing probably to dilute the hydrochloric acid gas. The next series con- 

 sisted of bulbs similarly prepared, but exposed for sixty-nine days (from 

 May 31 to August 7). From the analysis given in Table IT. it will be 

 seen that oxidation of the acid has taken place, even in presence of a small 

 excess of oxygen ; in many cases the whole of the acid has been oxidised 

 to chlorine and water, and in some cases hypochlorous acid (or some other 

 oxygen acid of chlorine) has been formed ; this accounts for the percentage 

 amount of free chlorine coming out too high. 



It appears probable that the oxidation takes place in two stages, the 

 first aotion of light being to oxidise part of the hydrochloric acid to hypo- 

 chlorous acid : this is at first decomposed into chlorine and water by the 

 excess of acid present, as is shown in the first series ; but when the greater 

 Tjart of the hydrochloric acid has been removed the hypochlorous acid 

 does not further suffer decomposition. 



Itate at luliicli oxidation takes place. — When gaseous hydrochloric 

 acid and oxygen are first exposed to light the decomposition goes on with 

 extreme slowness ; it rapidly increases, however, with the amount of 

 chlorine liberated. An experiment was made on this point in which a 

 tube containing hydrochloric acid and oxygen was exposed to light to- 

 gether with a tube containing a similar mixture, which had, however, been 

 previously exposed to light until 92*6 per cent, of free chlorine had been 

 set free ; when the chlorine was estimated in the two bulbs it was found 

 -that the first bulb contained 0*9 per cent, free chlorine, whereas the other 

 tube had gained 7 per cent., making a total of 99 per cent, free chlorine. 



Lifluence of free chlorine. — Experiments were made to determine what 

 influence free chlorine had on the decomposition of the acid. For this 



