December 30, 1921] 



SCIENCE 



655 



phosphoric acid in a solid state from solu- 

 tions containing certain impurities is another 

 case in point. 



Many of the salts which contain water of 

 crystallization, especially the sulphate carbon- 

 ate, and acetate of sodium, are remarkable 

 for the fact that when solutions are prepared 

 free from dust and under perfectly quiescent 

 conditions they refuse to crystallize far be- 

 low their ordinary point of saturation unless 

 they are disturbed by agitation, by particles 

 of dust or particularly by introduction of 

 crystals of the material, when they suddenly 

 crystallize throughout the entire mass. Jean- 

 nel years ago stated that he assumes "that 

 saturated solutions when heated form pecu- 

 liar hydrates and that these remain unaltered 

 when the temperature is lowered but that 

 vibration or the presence of a crystal of the 

 salt is sufficient to bring about their decom- 

 position." 



It has been observed by Ostwald that pre- 

 vious to the formation of sodium chloride 

 crystals iwints of congestion may be noted 

 where droplets form and can be seen under 

 the ultra microscope. This is analogous to 

 the suspensoid form of droplet of a colloidal 

 body, but while the colloidal droplet is stopped 

 from going further the sodium chloride drop- 

 let completes its course by coming together 

 into a crystalline mass with the complete ex- 

 clusion of tlie water of hydration which has 

 surrounded the molecule in its solution state. 

 Under conditions of supersaturation above 

 referred to it is supposed that these centers 

 of congestion are avoided and crystallization 

 does not occur until by agitation or the intro- 

 duction of nuclei such centers of congestion 

 are brought about. 



COLLOID SOLUTIONS 



( It would apjjear that the colloidal state of 



the silicon, aluminum, and iron is such, the 

 hydrates formed with the water molecules are 

 so complex and the internal energy of the 

 eystem is so low that the molecules of silicon, 

 aluminum, and iron, and the same would 

 probably be true for colloidal platinum, gold 

 and silver, are unable at ordinary tempera- 



tures to combine as a crystalline or amor- 

 phous mass. If the temperature is raised to 

 900 or 1000° the last traces of water dis- 

 appear, they lose completely their colloidal 

 properties and take on the form of an amor- 

 phous mass. 



The modern concept of the atom shows a 

 central nucleus charged with positive elec- 

 tricity surrounded by many electrons with 

 negative charges. If the two forces are per- 

 fectly balanced the material is inert as in 

 the case of nitrogen gas. The activity of the 

 atoms of other elements depends u]X)n the 

 extent to which this balance is thrown off 

 to one side or the other. 



From the modern concept of solutions it 

 would appear that the silicon, aluminum and 

 iron are completely dissociated in their col- 

 loidal solution but the positive and the negative 

 portions of the molecules are so balanced that 

 they are in an extremely inactive condition and 

 substances absorbed by them become equally 

 inert and inactive for the same cause and thus 

 change their state from a condition of true 

 solution to a colloidal solution. 



The principles of dyeing appear to be based 

 upon a similar change of state. The dye in 

 true solution enters a colloidal membrane such 

 as silk or wool, is changed to a colloidal state 

 in and on the membrane from which it can not 

 thereafter be dislodged with water as the col- 

 loidal solution is immiscible with water. The 

 principles of dyeing seem to rest upon the 

 ability of certain materials including mordants 

 which are of such conditions that they have the 

 power to bring about this change of state. 



The colloidal material separated from soils 

 in a dilution not exceeding one gram per liter 

 after thorough agitation appears under the 

 ultra microscope as minute di-oplets showing 

 points of congestion such as precede the forma- 

 tion of salt crystals and appear as droplets of 

 fat suspended in milk or as fog appears in the 

 cloud. In larger concentrations these droplets 

 coalesce into larger masses as the fat globules 

 coalesce on rising into cream or as the droplets 

 of fog coalesce to form the liquid water of 

 larger drops. Fog follows neither the laws of 

 gases nor the laws of liquids. The soil colloid 



