EXCEPTIONAL PHENOMENA. 325 



subject to these curious transmutations. Sometimes a 

 crystal embedded in a matrix is entirely dissolved away, 

 and subsequently a new kind of mineral is gradually 

 deposited in the cavity as in a mould. Quartz is thus 

 found cast in many forms wholly unnatural to it. A 

 still more perplexing case sometimes occurs. Carbonate 

 of lime is one of the substances capable of assuming 

 two distinct forms of crystallization, in which it bears 

 respectively the names of calcite and arragonite. Now 

 arragonite, while retaining its outward form unchanged, 

 may undergo an internal molecular change into calcite, 

 as indicated by the altered cleavage. Thus we may come 

 across crystals apparently of arragonite, which seem to 

 break all the laws of crystallography, by possessing the 

 cleavage of an entirely different system of crystallization. 



Some of the most invariable and certain laws of nature 

 are disguised by interference of unlooked-for causes. 

 While the barometer was yet a new and curious subject 

 of investigation, its theory, as stated by Tomcelli and 

 Pascal, seemed to be contradicted by the fact that in 

 a well-constructed instrument the mercury would often 

 stand far above 3 1 inches in height. Boyle showed h 

 that the mercury could be made to rise as much as 75 

 inches in a perfectly cleansed tube, or about two and a 

 half times as high as could be due to the pressure of 

 the atmosphere. Many absurd theories about the pres- 

 sure of imaginary fluids were in consequence put forth 1 , 

 and the subject was involved in much confusion until 

 the adhesive or cohesive force between glass and mercury, 

 when brought into perfect contact, was pointed out as 

 the real interfering cause. 



Guy-Lussac, again, observed that the temperature of 

 boiling water was veiy different in some kinds of vessels 



h 'Discourse to the Royal Society,' 28th May, 1684. 

 ' Robert Hooke's ' Posthumous Works,' p. 365. 



