G72 GLASS 



glass in a comparatively short time. From tho nature of the material, it becomes 

 necessary to divide the analysis into two distinct portions; one of which has for its 

 object the estimation of its alktiline ingredients, the other that of the earthy, metallic, 

 and siliceous matters. Having heated a sufficient quantity of tho sample in question 

 to dull redness, it must bo suddenly thrown, whilst still hot, into a basin containing 

 cold water. In this way it becomes cracked and flawed in all directions, so as to 

 favour its reduction into powder. When dry it must, therefore, be carefully ground 

 in an agate or steel mortar, until it has tho appearance of fine flour. Nor is it a 

 matter of indifference whether this takes place in contact with water or not ; for glass 

 in this extreme state of comminution readily gives up a part of its alkali to water ; 

 and hence, if ground in the presence of that fluid, tho resulting analysis would prove 

 incorrect. But we will suppose that a quantity of finely-powdered glass has been ob- 

 tained as above indicated, and the amount of its alkali is desired ; then weigh out 

 100 grains of the glass, and carefully mix with it 200 grains of pure fluor-spar in a 

 similarly-powdered condition. Place tho mixture in a platinum or leaden vessel, and 

 pour over it 500 grains of strong sulphuric acid, stirring the whole well together 

 wjth a silver spoon, but taking care not to remove any portion of the materials. 

 Next, apply a heat of about 212 Fahr. ; and as the process draws to a conclusion, 

 this may be raised as high as 300. When all evolution of gaseous fumes has ceased, 

 water may be poured on the residuary mass to the extent of four or five ounces, and 

 the mixture thrown on a filter. After the clear fluid has passed through, a little more 

 water must be added to the filter, so as to wash out the whole of tho soluble matter ; 

 these washings being joined to the original clear fluid, which consists of sulphate 

 of soda or potash, or both, with a quantity of sulphate of lime, and perhaps also of 

 magnesia and alumina. To this an excess of carbonate of ammonia must now be 

 added, to admit of the separation of the earthy salts being effected by filtration. The 

 clear solution is next boiled down to dryness, and the residue is heated red hot for a 

 minute or two. This residue is the soda or potash, or both, formerly contained in 

 100 grains of the glass, but now united to sulphuric acid. Having ascertained its 

 weight, the relative proportions of potash and soda may be found by testing its con- 

 tent of sulphuric acid with a barytic solution, and calculating the result by the well- 

 known Archimedean equation ; or, by dissolving the mixed salt in a small quantity of 

 water, and after adding an excess of tartaric acid, leaving the whole for a few hours 

 covered up in a cool place. Almost the whole of the potash will separate in this way 

 as bitartrate of potash. The quantity of alkali may be determined from the atomic 

 constitution of the alkaline salts. Thus, supposing the dry residue altogether com- 

 posed of sulphate of soda, then as 72 grains of it indicate 32 of pure soda, the result 

 may be obtained by the rule of proportion. The amount of alkali being known, 

 another portion of the powdered glass must be employed for ascertaining the re- 

 mainder of the ingredients. That is to say, 1 00 grains of the sample must bo mixed 

 with 200 grains of pure potash, and the whole fused together in a silver crucibh 1 , at a 

 red heat, until perfect liquefaction ensues, when the crucible and its contents may bo 

 withdrawn from the fire, and, as soon as cool enough, boiled in half a pint of pure 

 water, so as thoroughly to dissolve the fused mass from the crucible. An excess of 

 nitric acid being poured into the solution, the mixture is then evaporated to dryncss, 

 by which means the silicic acid is rendered insoluble ; consequently, on tho applica- 

 tion of water, this remains, and may be dried and weighed, whilst the lime, alumina, 

 and lead of the glass may be separated from the soluble portion by the addition, first, 

 of sulphuretted hydrogen, which separates the lead, then of ammonia, which throws 

 down the alumina, and, next, by pouring in carbonate of ammonia, which precipi- 

 tates the lime as a carbonate. Thus, therefore, the alkaline matters are found by one 

 process, and the silica, earthy, and metallic constituents by another, both of which may 

 be conducted at the same time. It has been recommended to employ carbonate of 

 baryta in the analysis of glass; but the high temperature required with this sub- 

 stance dissipates a portion of the alkaline components, and thus leads to serious errors. 

 Even mere fusion in a glass furnace expels soda from glass, and renders it r\\- 

 more infusible ; but this expulsion is much favoured by tho presence of baryta. Tho 

 above method of analysing glass is. t lien-tore, to be preferred to the baryta, plan, by 

 individuals not habitually engaged in manipulative chemistry. Ure. 



Glass, for horticultural purposes. An impression taken up loosely in tho iirst 

 instance from some experiments on the action of the chemical rays of light, when 

 made to permeate coloured glass, has led the public frequently to conceive that 

 glasses which admitted freely the chemical rays wero tho most adapted to accelerate 

 the growth of plants. No more mistaken view was ever entertained. At different 

 periods in the life of a plant different influences are necessary ; at one time tho 

 chemical force is required, at another the luminous power, and at another ihr> calorific 

 Tin; i^olar rays, as we rocuve them direct from the bun, have those t'oive.-* 



