HOW MOLECULES MAKE MASSES 45 



paradoxical though the statement may appear. When ice is formed at 

 temperatures a few degrees under 0° C, it has a well-marked crystalline 

 structure, as is seen in water frozen from a state of vapor in the form 

 of flakes of snow and hoar-frost, or in water frozen from dilute sul- 

 furic acid, as observed by Mr. Faraday. But ice formed on contact 

 with water at 0° C is a plain, homogeneous mass with a vitreous frac- 

 ture, exhibiting no facets or angles. This must appear singular when 

 it is considered how favorable to crystallation are the circumstances in 

 which a sheet of ice is slowly produced in the freezing of a lake or 

 river. The continued extraction of latent heat by ice as it is cooled 

 a few degrees below 0° C, observed by Mr. Persons, appears also to 

 indicate a molecular change subsequent to the first freezing. 



"Further, ice, although exhibiting none of the viscous softness of 

 pitch, has the elasticity and tendency to rend seen in colloids. In the 

 properties last mentioned it suggests a distant analogy to gum incom- 

 pletely dried, to glue or any other firm jelly. 



"Ice further appears to be of the class of adhesive colloids. The 

 reintegration (regelation of Faraday) of masses of melting ice when 

 placed in contact has much of a colloidal character. The colloidal 

 view of the plasticity of ice demonstrated in glacial movement will 

 readily develop itself. 



"A similar extreme departure from the normal appears to be pre- 

 sented by a colloid holding so high a place in its class as albumen. In 

 the so-called blood-crystals of Funke, a soft gelatinous, albumenoid 

 body is seen to assume a crystalline contour. 



"Can any facts more strikingly illustrate the maxim that in nature 

 there are no abrupt transitions, and that distinctions of class are never 

 absolute?" 



It is of interest to note that the word "crystal" arose from the notion 

 that rock crystal (clear, transparent quartz obtained from the Alps in 

 ancient times), had been formed from water by intense cold. The 

 word is derived from the Latin crystallum (clear ice) or the Greek 

 krystallos, (from kryos, frost). Not until the 17th century was the 

 word extended to the crystals familiar to all of us. In fact, the Romans 

 called many crystals glasses (vitrum), and this survives in such expres- 

 sions as blue vitriol (copper sulfate crystals), white vitriol (zinc sulfate), 

 and green vitriol (ferrous sulfate). The corrosive, oily liquid obtained 

 by distilling "green vitriol" was called "oil of vitriol." 



While there is no space here to consider the experimental and 

 theoretical aspects of crystals and crystallization, a few popular 

 misconceptions should be set straight. First, Thomas Graham, 

 the father of colloid chemistry, was well aware of the fact that the 

 same substance may exist either in the colloidal or the crystalloidal 



