THE BEGINNINGS OF LIFE. 59 



prisms — mixed with a few crystals of calc-spar, because 

 the temperature of the solution is lower at first than it 

 afterwards becomes.' In other examples we may find 

 not only a striking difference in physical form, but 

 also a notable change in colour_, occasioned by the new 

 molecular rearrangements which the change of tem- 

 perature seems to necessitate: — ^Protoxide of lead 

 crystallizes' after fusion, as well as from a saturated 

 solution in hot concentrated caustic potash, in yellow 

 rhombic octahedrons. If, however, the solution is not 

 fully saturated with oxide of lead, so that crystallization 

 does not take place till after complete cooling, red 

 crystalline scales are deposited on the yellow rhombic 

 octahedrons just formed : if the red crystals are heated, 

 they turn yellow in cooling, in consequence of passing 

 into the first form.' 



But in addition to these variations in which a crys- 

 talline form of one kind or another is always present, 

 we may occasionally find that in saline solutions, solid 

 products make their appearance whose outlines are 

 rounded instead of being bounded by right lines and 

 angles. Thus, Dr. Hughes Bennett^ has figured a pecu- 

 liar pellicle that fjjrms on the surface of lime-water, 

 in which the crystalline material very closely resembles 

 a layer of tesselated epithelial cells. And lately I have 

 seen a somewhat similar crystalline pellicle make its 

 appearance on the surface of a solution ijn 'vacuo) 

 containing small quantities of ammonic sulphate and 



^ 'Lancet,' 1863, vol. i. p. 3. 



