Volume 2

A system of chemistry / by Thomas Thomson.

  • Thomson, Thomas, 1773-1852.
Date:
1820
Catalogue details

Licence: Public Domain Mark

Credit: A system of chemistry / by Thomas Thomson. Source: Wellcome Collection.

Provider: This material has been provided by King’s College London. The original may be consulted at King’s College London.

    732/740 (page 720)
    GENERAL REMARKS. mS£J!h Wh'Ch °il haS t0 combine with ox)rgen when raised to a high * temperature. 3d Process The third method is most frequently practised, because it is not liable to the same inconveniences as the other two. This method consists in boiling the oil and the oxide together in a sufficient quantity of water. By this liquid the heat is mo- derated at first till the oil and oxide combine, which prevents the revival of the metal; and afterwards when the water is dis- sipated, the temperature is sufficiently high to give the plaster the requisite consistency. Plasters, when long kept, become often too hard to be fit for use, especially if the requisite proportion of oil has not been employed at first. This defect is easily remedied, by melting them with a small portion of new oil. Plasters, when long kept, likewise change their colour, and most of their sen- sible properties: owing either to the absorption of oxygen, or to some change produced in their component parts by the ac- tion of the air. CHAP. V. GENERAL REMARKS. If we exclude the compound combustibles, the nature of which has not been hitherto sufficiently studied, all the pri- mary compounds, with a few exceptions indeed, may be divided into the two grand classes of acids and bases. All the secon- dary compounds described in the preceding chapters, except the hydrates, may be considered as combinations of acids and bases. The hydrates indeed have been considered by some modern chemists, as likewise saline combinations. But this name cannot be applied to them without extending the mean- ing of the terms acid and base so far as to render them really useless. For in the hydrates we must consider the /cater as sometimes acting the part of an acid, and sometimes of a base. While in the crystallized salts, as in sulphate of' soda, it would be impossible either to apply the term acid or base to the water without manifest impropriety. It seems much better, there- fore, to consider hydrates as a species of compound quite dif- ferent from sails. The water is capable equally of uniting with acids, bases, and salts. It docs not deprive them of their cha- racteristic properties, or neutralize them as the bases do the