Although at present in commerce the strength of potashes is frequently guaranteed, it is still desirable to estimate the strength. It is sufficient for practice to allow a small quantity, say 100 grammes, to stand with an equal quantity of very cold water for some hours, then to filter and pour a similar quantity of water over the residue on the filter. The weight of undissolved substance subtracted from 100 gives with sufficient accuracy the weight of pure potassium carbonate contained in 100 parts of potashes. This method is founded on the fact that potassium carbonate dissolves readily even in cold water, but the other salts with difficulty. This procedure can also be used to obtain pure potassium carbonate from crude potashes; it is only necessary to filter and evaporate to dryness the solution obtained by pouring very cold water on crude potashes. =Potassium Hydroxide= (Potassium Hydrate, Caustic Potash), KOH = 56.—The commercial variety consists of very deliquescent white lumps, generally containing a large quantity of impurities. On this account caustic potash, or rather a solution of it, is prepared in the colour works. With this object 11 parts of potash, contained in a tub with an opening at the bottom, are mixed with 100 parts of cold water. Two hours later, the clear solution is run off into a clean iron pan, in which it is heated to boiling. To the boiling solution is added milk of lime prepared from water and 3·5 parts of quicklime. After the liquid has boiled a few minutes, a small portion is filtered and hydrochloric acid added to the clear filtrate; if no effervescence occurs, then all the potassium carbonate is converted into caustic potash. Should effervescence occur, milk of lime is added until a new portion no longer effervesces on the addition of hydrochloric acid. Then the pan is covered with a well-fitting lid, and the cooled liquid, if not required for immediate use, preserved in well-corked glass bottles. The strength of a caustic potash solution can be found by means of a hydrometer. The following table shows the relation between the specific gravity of a solution and the percentage of caustic potash it contains:— +----------+----------------+ | Specific | Caustic Potash | | Gravity. | per cent. | +----------+----------------+ | 1·06 | 4·7 | | 1·11 | 9·5 | | 1·15 | 13·0 | | 1·19 | 16·2 | | 1·23 | 19·5 | | 1·28 | 23·4 | | 1·39 | 32·4 | | 1·52 | 42·9 | | 1·60 | 46·7 | | 1·68 | 51·2 | +----------+----------------+