In numerous experiments, we are obliged to use tubes of glass, porcelain, or copper, instead of gun-barrels; but glass has the disadvantage of being easily melted and flattened, if the heat be in the smallest degree raised too high; and porcelain is mostly full of small minute pores, through which the gas escapes, especially when compressed by a column of water. For these reasons I procured a tube of brass, which Mr de la Briche got cast and bored out of the solid for me at Strasburg, under his own inspection. This tube is extremely convenient for decomposing alkohol, which resolves into charcoal, carbonic acid gas, and hydrogen gas; it may likewise be used with the same advantage for decomposing water by means of charcoal, and in a great number of experiments of this nature.
[60] The representation of this apparatus, Pl. IV. Fig. 1. will convey a much better idea of its disposition than can possibly be given by the most laboured description.--E.
_Of the Composition and Application of Lutes._
The necessity of properly securing the junctures of chemical vessels to prevent the escape of any of the products of experiments, must be sufficiently apparent; for this purpose lutes are employed, which ought to be of such a nature as to be equally impenetrable to the most subtile substances, as glass itself, through which only caloric can escape.
This first object of lutes is very well accomplished by bees wax, melted with about an eighth part of turpentine. This lute is very easily managed, sticks very closely to glass, and is very difficultly penetrable; it may be rendered more consistent, and less or more hard or pliable, by adding different kinds of resinous matters. Though this species of lute answers extremely well for retaining gasses and vapours, there are many chemical experiments which produce considerable heat, by which this lute becomes liquified, and consequently the expansive vapours must very readily force through and escape.
For such cases, the following fat lute is the best hitherto discovered, though not without its disadvantages, which shall be pointed out. Take very pure and dry unbaked clay, reduced to a very fine powder, put this into a brass mortar, and beat it for several hours with a heavy iron pestle, dropping in slowly some boiled lintseed oil; this is oil which has been oxygenated, and has acquired a drying quality, by being boiled with litharge. This lute is more tenacious, and applies better, if amber varnish be used instead of the above oil. To make this varnish, melt some yellow amber in an iron laddle, by which operation it loses a part of its succinic acid, and essential oil, and mix it with lintseed oil. Though the lute prepared with this varnish is better than that made with boiled oil, yet, as its additional expence is hardly compensated by its superior quality, it is seldom used.