Labels written in indian ink on white paper can be rendered indelible by painting them, after the gum has dried, with a brush dipped in celluloid varnish or melted paraffin wax. If this is not done, a bottle should be held with the label upwards when pouring out liquids, so that if any of the solution should trickle down the sides the label will remain intact. 2 The labelling of all large receptacles should be done with paint.
273. Solutions, Concentration, Solubility, Saturation. A solid, liquid, or gaseous substance dissolves in a liquid when it disappears into the liquid, giving a homogeneous solution; the his solved substance is called the solute, and the liquid in which it is dissolved the solvent; evaporation of the solvent leaves the solute unchanged.
For practical purposes the concentration or strength of a solution may be taken as the weight in ounces of the solute in Ioo oz. of the solution. For example, if 20 oz. of hyposulphitc are dis solved in a certain quantity of water and, after solution, made up to ioo oz., then this solution is said to be 20 per cent, or a 20 per cent hypo sulphite solution. (When the solvent is water, the fact is not usually stated.) At any one temperature, a solvent will dissolve only a fixed amount of a salt, and when the solution has attained its maximum concentration it is said to be saturated. This concentration is known as the solubility at the temperature in question.
Except in very rare cases the solubility of a salt increases with rise in temperature. On cooling a saturated solution, the excess of the solute over the solubility at the lower tempera ture separates out in the form of crystals, which are generally much purer than the original substance (purification by crystallization), since the impurities present have not been able to reach their saturation point, as long as the amount present in the salt is not considerable.
In certain cases, however, a carefully cooled saturated solution will not deposit crystals, but will remain in unstable equilibrium. Such a solution, called supersaturated, will deposit the excess of salt in solution immediately, if a speck of the same salt (germ) is brought into contact with it.
The speed of solution depends on various factors. A porous or very finely divided sub stance will dissolve much more readily than one in the form of large compact lumps. Neverthe
less, water should not be poured on to a pow dered anhydrous substance, but the latter should be dropped into the water in small quantities to prevent the formation of a large compact mass of the hydrated salt. Solution takes place more rapidly in hot than in cold water, and is accelerated by agitation. Since the solution is denser than the solvent, there is a tendency for a saturated solution to be formed in the neighbourhood of the salt unless this is suspended in a porous pot or a bag at the top of the liquid.
274. Expression of Formulae. A correctly stated formula gives the substances in the order in which they are to be dissolved.
The quantities are expressed in weights of solid substances, or volumes (at 65° F.) for liquids (grammes and cubic centimetres for preference).
The weights or volumes are arranged to give a total volume of 20 07.. (or I,000 c.c.). The volume of the principal solvent need not be expressly stated, but may be indicated by the instruction," Sufficient quantity to make 20 oz." Or some such equivalent term.
When a formula denotes that a certain number of drops of a solution are to be measured, to avoid confusion the number is usually given in roman figures. It is, however, better to pre scribe the quantity in cubic centimetres or of a more diluted solution.
275. Water used in the Preparation of Baths. With the exception of distilled water, rain-water, and water melted from ice, the water usually at our disposal (tap, river, and well water) contains dissolved salts (chiefly bicarbonate and sulphate of lime), " suspended matters (dust, rust particles from the iron pipes), organic substances of animal and vegetable origin, which in colloidal form escape the most perfect filters, and after coagulation deposit in the baths as a fine mud, and, lastly, dissolved gases, notably oxygen and carbon dioxide.
Business establishments, possessing a water supply of poor quality, but which are able to obtain distilled water quite cheaply (condensed water from boilers) should certainly use the distilled water for the preparation of all baths, since the only impurities it contains are small quantities of dissolved gas.