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XXV. 'Experiments on the Congelation of the Vitriolic Acid. By 
James Keir, Efq. F, R . S. ; communicated by Henry Caven- 
difh, Efq. F. R. S. 
Read May 3, 1787. 
§ 
T HAT the vitriolic aci&Yometimes alfumes a folid, crystal- 
line State, has been obferved by Basil Valentine, and* 
by many later chemiSts ; but their relations of this appearance are 
neither fufhciently explicit, with regard to the effential and con- 
comitant circumStances, nor do they feem very confiStent with 
each other. It appears, however, that two very diftindt fpecies of 
congelation of this acid have been noticed. That which is 
defcribed by the older chemiSts, and alfo by fome modern 
authors, requires no greater degree of cold than the common 
temperature of the air, even in fummer, and is peculiar to 
that acid which is obtained by distillation from martial vitriol, 
and which is poffefled of a Smoking quality in a high degree : 
for not only the authors, by whom this congelation has been * 
obferved, have given this defcription of the acid employed, 
• but alfo the late experiments of M. Dollfuss * feem to 
Shew, that the Smoking quality is effential to the phenomenon; 
for neither the acid obtained from vitriol, when deprived by 
redtificaticn of its Smoking quality, nor the Englifh oil of * 
vitriol (which is known to be obtained by burning Sulphur, and i 
* Crell Annalen 1785. 
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