74 



CHEMISTRY. 



Klemtnti 

 of 



Chemistry. 



CHAP. II. 

 Of Glass. 



SILICA, when mixed with the fixed alkalies and ex- 

 posed to a strong heat, enters readily into fusion. It 

 melts also when heated along with some of the alka- 

 line earths, as lime, provided a little alumina be pre- 

 sent. These- mixtures are very ductile while in fu- 

 sion, and may be readily moulded into any shape we 

 please. If they be suddenly cooled below the tem- 

 perature at which they become solid, they retain 

 their transparency, and assume those peculiar proper- 

 ties which belong to the substance called glass. Glass, 

 then, is a combination of the fixed alkalies, or alka- 

 line earths with silica, either alone or conjoined with 

 alumina, brought into complete fusion, and then sud- 

 denly congealed. Metallic oxides are sometime i add- 

 ed ; they assist the fusion like the alkalies, and com- 

 municate frequently a peculiar colour to the vitreous 

 mass. 



When glass is in fusion, the substances which en- 

 ter into its composition may be considered as combi- 

 ned with each other, so as to form a homogeneous 

 mass similar to water holding a variety of Baits in so- 

 lution. If it be cooled down very slowly, the differ- 

 ent tendency of the constituents to assume a solid 

 form at peculiar temperatures, will cause them to se- 

 parate successively in crystals ; just as the salts held 

 in solution in water assume the form of crystals as 

 the liquid is slowly evaporated. But if the glass be 

 quickly cooled down to the point of congelation, the 

 constituents have not time to separate in succession, 

 and the glass remains the same homogeneous com- 

 pound as while in a state of fusion ; just as would 

 happen to a saline solution if suddenly exposed to a 

 cold capable of congealing it completely. Hence it 

 appears that the vitfeous quality depends entirely up- 

 on the fusibility of the mixture, and the suddenness 

 with which it is cooled down to the point of conge- 

 lation. The substance, though solid, is precisely the 

 same as to its chemical composition as if it were still 

 in fusion ; the sudden cooling having fixed the con- 

 stituents before they had time to assume a new ar- 

 rangement. 



All fusible mixtures of the earths proper with fix- 

 ed alkalies, alkaline earths, or metallic oxides, may 

 be made at pleasure to assume the form of glass, or 

 the appearance which characterises stone or porce- 

 lain, according to the rate of cooling ; and glass may 

 be deprived of its vitreous form, merely by fusing it 

 and cooling it down with sufficient slowness to ena- 

 ble the constituents to separate in succession. Sir 

 James Hall found that glass (consisting of various 

 earthy bodies) always loses its vitreous state, and as- 

 sumes that of a stone, if more than a minute or two 

 elapses while it is cooling down from complete fusion 

 to the point at which it congeals. 



There are different kinds of glass in common use 

 for various purposes. The finest are plate glass, and 

 JHnt plats or crystal. They are perfectly transparent, 

 nt.'i.y colourless, heavy, ai.d brilliant. They arc 

 composed of fixed alkali, pure siliceous sand or calci- 

 ed fliatt, and litharge. Crown glass is made with- 



out lead ; it consist! of fixed alkali and siliceous sand, Elements 

 and is much lighter than flint glass. It has a dibtinct 

 greenish tinge, from the oxide of iron present in the 

 materials employed in making it. Sometimes too 

 great a proportion of oxide ol manganese i added, 

 which gives it a purple colour. Bottle glass is the 

 coarsest and cheapest kind. It consists chiefly of 

 lime fiivil with silica and a little alumina, and con- 

 tains s;> much iron and manganese as to give it a dark 

 colour, and to diminish its transparency very much. 

 It is much harder, stronger, and more difficultly fu- 

 sible than the fine kinds of glass. 



Glass answers well as a chemical vessel, as it is 

 acted on only by a small number of re-*gents. Fluoric 

 acid corrodes it readily ; so do the fixed alkalies when 

 assisted by heat. Water, when long boiled in it, dis- 

 engages some alkali from it, and occasions the sepa- 

 ration of silica in the state of a white powder. 



CllAP. III. 



Of Sails. 



THE word sail was originally confined to common 

 tall, a substance which has been known and in com- 

 mon use from the remotest ages. The term was af- 

 terwards generalized by chemists, and applied to all 

 bodies which are sapid, easily melted, soluble in wa- 

 ter, and not combustible. At length it was confined 

 to acids, alkalies, and the combinations of these bo- 

 dies with each other. At present the term is applied 

 to all the compounds which the acids form with alka- 

 lies, earths, and metallic oxides. 



Chemists have agreed to denominate the salt from 

 the acids which they contain. The alkali, earth, or 

 metallic oxide, combined with that acid, is called the 

 base of the salt. Thus Glauber salt, being a com. 

 pound of sulphuric acid and soda, is called a sulphate, 

 and soda is called the base of Glauber salt. Hence 

 it follows, that there are as many genera of salts as 

 there are acids, and as many individual salts, or spe- 

 cies, as there are combinations of acids with a base. 

 Silica and some of the metallic oxides do not appeal- 

 capable of combining with acids. But to compen- 

 sate this, there are some acids which combine with 

 two bases at once, and form what are called triple 

 salts. Thus tartaric acid combines at once with pot- 

 ash , and soda. Some salts combine with an addi- 

 tional dose of their acid, and others with an addition- 

 al dose of their base. The first render vegetables 

 blue, the second usually render them green. The 

 first kind of salts are distinguished by prefixing to 

 the usual name the preposition sit/jfr, the second by 

 prefixing the preposition sub. Thus sulphate of pot- 

 ask, denotes the salt in a state of perfect neutrali/a- 

 tion, without any excess either of acid or potash ; 

 Mtpefsitlphate o/' /inla^'i is the same salt with an excess 

 of acid; lubiittpntttt of potash is the same salt with 

 an excess of !>.. 



As the different genera are denominated from the 

 acids, it is obvious that tlnv niu.-t be as many gene- 

 ra as there are acids. The termination of the names 

 of these genera differs according to the acid which 

 constitutes them. When the acid contains a jnaxi- 

 mum of oxygen, the termination of the genus is ate ; 



