1. It Should Be Free From Injurious Matter And Hurtful Organisms

Among these we find decomposing organic (vegetable and animal) matter, which is often derived from sewage; and infective microbes, which play a large part in the spread of disease in the tropics, and a smaller though not an insignificant one in temperate climates. Organic matter in drinking water is of far less consequence of itself than the fact that it renders water a favourable medium for the cultivation of disease germs; pure water being altogether unsuitable for that purpose. We should not accept the presence of fish in water as an unquestionable proof of its practical purity; because many species of coarse fish are able to thrive in water which is unfit for horses to drink. In India and other hot countries, stagnant water often contains large numbers of thread-like worms (filarioe), which produce various forms of disease among horses. Neumann gives in his book on the parasites of domestic animals (which has been translated by Fleming) a full account of the serious effects on horses caused by leeches entering their mouths while the horses are in the act of drinking. The leeches (chiefly the Hoemopis sanguisuga) which make this attack are generally young ones which usually inhabit running water, and are seldom longer than 1/50th of an inch. The streams and springs of Algeria are greatly infested by them.

2. It Should Be Sufficiently Aerated

When water is freely exposed to the air, as in the case of rain, or when water is poured backwards and forwards from one vessel into another, it will absorb a certain proportion of the gases which are in the atmosphere. Thus, a gallon of freshly fallen rain water will generally contain about 4 cubic inches of nitrogen, 2 cubic inches of oxygen, and 1 cubic inch of carbonic acid gas (Bloxam). We here see that oxygen is more soluble than nitrogen in water; because air contains about four volumes of nitrogen to only one of oxygen. The absorbent power of water for gases diminishes in proportion as its temperature is raised, as we may prove by bringing into a warm room a glass jug full of fresh rain water, in which case bubbles of gas which have separated themselves from the water, will be seen to quickly form on the inside of the jug. When stagnant rain water is exposed to the rays of the sun, it will, on becoming heated, lose a certain percentage of its dissolved gases, and will be far less capable of recovering this loss than running water, on account of the comparatively small surface it presents to the atmosphere. Hence we find that rapidly running shallow streams, especially if their course is a good deal broken up by inequalities in their bed, are more highly aerated than stagnant collections of rain water. Under the ordinary pressure of the atmosphere, one volume of water will dissolve about one volume of carbonic acid gas. With a pressure equal to that of two atmospheres, it will dissolve two volumes; with a pressure of three atmospheres, three volumes; and so on. Spring water, when below the surface of the ground, often becomes charged with carbonic acid gas under pressure. When this pressure is removed by the water rising to the surface, the extra carbonic acid escapes with more or less effervescence. This gas imparts to the water a bright appearance and pleasant taste, which are in marked contrast with the dull look and insipid flavour of soft water. The gases contained in naturally aerated water promote digestion by, among other ways, assisting the digestive juices to permeate through the food that is in the alimentary canal. The fact that this water is universally found by mankind to be pleasanter to drink than water poor in gases, is a strong proof that it is the more healthy beverage of the two. Although from want of exact knowledge we cannot unreservedly extend this conclusion to horses, the probabilities are all in favour of its being equally true in their case.

The foregoing considerations suggest the sound practical rule that it is better to give a horse fresh water, than water which has been allowed to stand for some time after it has been drawn, and that it should not be artificially warmed. If a particular sample of water happened to hold a large quantity of suspended matter, as might occur on rare occasions, it might be necessary to let the solid particles settle down as sediment, before giving the water to a horse to drink.

The presence of carbonic acid gas in water increases the solubility of carbonate of lime and may thus assist in nutrition.

3. It Ought Not To Contain An Excess Of Mineral Matter

3. It ought not to contain an excess of mineral matter, which excess may be defined as a greater quantity than 4 1/2 grains to a pint of water. The Thames water which is supplied to London contains about 2 3/4 grains to the pint. The principal mineral substances held in solution by river and spring water are carbonates of lime, soda, and magnesia; sulphates of soda, potash, lime, and magnesia; nitrates of potash, soda, and magnesia; chlorides of sodium and lime; and peroxide of iron. The chief mineral matters held in suspension are clay and sand.

Drinking water may be broadly classed as soft water, and hard water, which is the term commonly applied to water in which it is difficult to make a lather with soap. Hardness of water is caused by a comparatively large percentage of mineral matter being held in solution in that fluid, and may be divided into temporary hardness and permanent hardness. Temporary hardness is due to the presence of carbonate of lime and to a lesser extent to that of magnesium carbonate, both of which are soluble in water containing carbonic acid. Hence, when temporary hard water is boiled, the carbonic acid is driven off and the hardness disappears by reason of the precipitation of the carbonate or carbonates, as the case may be. Permanent hardness is caused by salts which are not precipitated by boiling. The best example of soft water is distilled water, and, after that, rain water and river water. Spring water and well water are generally hard. Mr. Thomas Hawkesly, the engineer, points out that at least So per cent. of the surface of the globe yields hard water. Letheby considers that moderately hard water is best for the drinking, purposes of human beings. My experience leads me to form the same opinion of hard water for horses, provided that the hardness is temporary. " The presence of dissolved solid matter in the water also influences its taste, preference being generally expressed for those waters which are not exceedingly poor in such solids" (Bloxain). Besides, carbonic acid gas is always an agreeable addition to drinking water, and as carbonate of lime is readily soluble in water containing that gas, it can hardly fail to be present in natural water in which that gas is dissolved. All natural water which is palatable to human beings, and most probably to horses, is hard. The preference sometimes evinced by horses for muddy water is, I think, due as a rule to deficiency of mineral matter in the food of these animals, and if that be the case, it cannot be reasonably urged as an argument in favour of the employment of such water for the drinking purposes of horses which are kept under healthy conditions of food. The deficiency in question is usually that of common salt. When water is brought by leaden pipes, there is less danger of lead poisoning with water which contains carbonate of lime in solution, than with soft water; because the oxide of lead which is formed by the action of the oxygen in the water, is practically insoluble in water that contains a fair quantity of carbonate of lime (Bloxam).