12 SCIENTIFIC AMUSEMENTS. 



circumstances firstly, that the cement, &c., adheres firmly to the 

 surfaces to be united ; and secondly, that as it sets or dries this 

 adhesion still continues, whilst the solidification of the cement or 

 solder, or the drying up of the paste or glue, gives sufficient 

 cohesion to the film of uniting material to prevent its separating 

 in the way that two pieces of wood would do if freshly glued 

 together, and moved before the glue is dry, or two pieces of metal 

 soldered together but handled before the solder has set hard by 

 cooling. In all cases where two solid objects are thus cemented 

 together, the strength of the junction depends on the degree of 

 cohesion of the cementing material and its adhesion to the surfaces 

 of the solids to be united ; if either of them be weak, the joint 

 will bear but little rough usage. Thus glass and wood cannot 

 be effectively glued together ; a strip of wood glued to a window or 

 mirror will easily break away, the rupture usually occurring at the 

 surface of the glass, the glue parting therefrom owing to the small 

 adhesion between glass and glue. On the other hand, two flat 

 pieces of wood well glued together and then forcibly separated 

 will frequently not divide along the line of the glue ; one piece of 

 wood will often split sooner than this will happen, leaving part 

 still glued to the other piece. It is possible to split a sheet of 

 paper (such as a bank-note) into two by cementing it firmly 

 between two flat surfaces and then forcibly separating them ; the 

 cohesion of the paper is less than its adhesion to the film of 

 cement, so that the paper splits before the junctions will yield. 



Changes of State produced by Variation of Temperature. 



One of the commonest observations in nature is that liquid 

 water, when subjected to considerable cold, as in winter time, 

 changes its physical condition of fluidity to that of solidity, the 

 mobile liquid freezing to a mass of ice, which melts again to 

 liquid water on warming ; whilst on the other hand, when heated 

 in an appropriate vessel, water evaporates or disappears from 

 view, becoming transformed into an invisible vapour somewhat 

 resembling the air and readily miscible therewith, but unlike air 

 being capable of reappearing in the liquid form on slightly cooling 

 again, making its appearance as mist, dew, or larger visible drops 

 of water, according to the circumstances under which the cooling 

 takes place. 



Experience teaches us that other bodies besides water exhibit 

 the same properties; by the application of heat of sufficient 

 intensity, almost all known solids can be melted (i.e., transformed 

 into liquids), and most known liquids can be vaporised (i.e., trans- 



