356 Chemical Observations. 



both Henry and Turner ; but on washing that gas" more sedu- 

 lously with caustic potash, its interference was found to depend 

 on a trace of impurities, for the ball always acted on explosive 

 mixture within a few minutes, however largely diluted with this 

 gas, if properly purified. Indeed, I had frequent occasion to 

 separate hydrogen from olefiant gas, and found cold spongy 

 platinum most effectual for the purpose. Neither did sul- 

 phuric acid vapour retard the action of the platinum ; indeed, 

 the other allowed the action to go on so rapidly, that from the 

 elevation of temperature it was itself slightly acted upon, car- 

 bonic acid always appearing. The same was the case with the 

 vapours of naphtha and the essential oils. 



The action of these gases here, therefore, is unlike the action 

 of the same gases and vapours in protecting phosphorus from 

 oxidation. 



The influence of sulphurous acid and sulphuretted hydrogen 

 is not impaired by diminishing the barometric pressure. 



2. Crystallization of Barley-Sugar. 

 The change in appearance, arising from crystallization, which 

 sticks of barley-sugar undergo in keeping, is always instanced 

 as a case of crystallization occurring in a solid body, without 

 solution, and independently of external agents. The barley- 

 sugar certainly does not then become a hydrate ; and probably 

 at the completion of the change is exactly of the same weight 

 as before it began. But from an observation I have made, it 

 would appear that the presence of a little moisture is necessary 

 for the change, and probable that every portion of barley- 

 sugar which suffers this change has been successively loosened 

 and held in solution by that small portion of water, which 

 begins to act on the outer surface of the stick and travels 

 inwards. 



Two fresh sticks of barley-sugar, dry and transparent, were 

 introduced at the same time into separate phials; one of them 

 with a stick of caustic potash, and the other by itself, corked 

 up, and laid in a drawer. The barley-sugar, in company with 

 the caustic potash, which would preserve it perfectly dry, 

 did not undergo the slightest alteration in six months, but 

 remained as transparent as at first. The barley-sugar in 



