Febbuaby 5, 1909] 



SCIENCE 



207 



chemical behavior of ionic substances, and 

 especially the hydrolyzing action of ionized 

 water, are other factors in the problem 

 under consideration. The relative strength 

 of chemical activity in the base-forming, 

 or acid-forming, elements, and their ability 

 to form acids and salts, lead to the discus- 

 sion of the production of the pyrogenetic 

 minerals from liquid magmas composed of 

 elements found in igneous rocks; some of 

 these minerals having been produced in the 

 laboratory by melting together the com- 

 ponent elements in proper proportions. 



Considering what should take place in a 

 solution having the composition of an 

 average of all igneous rocks, it can be 

 shown, since the chief acid-forming ele- 

 ments present are silicon in large amount, 

 and the more active element phosphorus 

 in very small amount, that salts with these 

 elements in the acid radical must be com- 

 mon. Other acid-forming elements occur- 

 ring in small amounts are titanium and 

 zirconium; while iron and aluminium may 

 play this role under favorable conditions. 

 The more active, phosphoric, acid forms 

 unstable salts with the active base-forming 

 metals, potassium and sodium, but a very 

 stable compound with the less active metal, 

 calcium, into which compound fluorine, or 

 chlorine, enters ; yielding apatite, an almost 

 universal component of igneous rocks. 



Silicon is known in the laboratory to 

 form one definite acid, H^Si04, orthosilicic 

 acid; and other acids of silicon have not 

 been isolated and identified. But very 

 definite mineral compounds exist that indi- 

 cate that salts from other silicic acids form 

 under proper conditions. These are: 



HiSiOi, orthosilicic acid; 



H^SiOs, metasilicic acid; 



HjSiaOs, polysilicic acid; 



HjSijOs, disilicic acid. 



It is significant that in laboratory experi- 

 ence with orthosilicic acid, H^SiO^, pre- 

 pared from aqueous solutions, the com- 



pound may be made to lose water gradually 

 until nothing but silica, SiOa, remains. In 

 this way free silica may be separated from 

 a silicate compound, a hydrogen silicate; 

 since an acid may be considered as a hydro- 

 gen salt. 



Observations upon the pyrogenetic min- 

 erals, and laboratory experience with syn- 

 thetical operations, show that salts of sev- 

 eral kinds of silicic acids form by the side 

 of one another, and that their character and 

 amount depend on the nature of the base- 

 forming elements present in the mixed 

 solution. Orthosilicates, metasilicates and 

 polysilicates commonly form in the pres- 

 ence of 'one another, sometimes accom- 

 panied by uncombined silica. And it be- 

 comes more and more evident that the 

 formation of the different kinds of silicic 

 acid ions, or their salts, is controlled pri- 

 marily by the strength, or chemical activ- 

 ity, of the base-forming elements; is de- 

 pendent also on the amount of silica avail- 

 able in the solution; and may be modified, 

 of course, by other factors. Thus it ap- 

 pears that the most active metals command 

 the highest silicic ions, the highest silicates 

 common in igneous rocks being the poly- 

 silicates of the alkalies, potassium and 

 sodium— orthoclase and albite. 



Further, the abundance of aluminium in 

 most rock magmas resiilts in the presence 

 of abundant aluminous compounds. And 

 this element, which is relatively inactive 

 chemically, being found sometimes in the 

 basic, sometimes in the acid radical, is 

 oftenest combined with the strongest base- 

 forming elements, potassium and sodium. 

 These relations are illustrated by the fol- 

 lowing common, simple pyrogenetic min- 

 erals. 



It is well known that the orthosilicate 

 of sodium and aluminium (nephelite) and 

 the metasilicate of potassium and alumin- 

 ium (leucite) do not form in the presence 

 of free silica (quartz), while metasilicates 



