4. Value of Water to Animals and Plants.-Nothing need be said about the value of water in the household. Life itself is impossible without it, to say nothing of the comfort it brings. Digestion is merely a process by which solid foods are made soluble that they may be carried through the blood to all parts of the body. As water is the most nearly universal solvent it enters largely into the process of digestion. Assimilation of food is impossible in the absence of water; and not only is this true, but most of the waste matter of the body is carried away dissolved in water or mixed with it in large amounts. On account of its high specific heat, that is, a given amount of water contains more heat energy than the same weight of any other liquid at the same temperature, it is regarded as the best means of warming houses in severe weather. It is this very fact that tempers the winters in the Great Lake regions and along the oceans, and renders the climate of countries washed by the Gulf Stream and Japan Current far warmer than other countries in the same latitude not thus favored. In the human body, the blood, largely water, in constant circulation, tends to keep the body of perfectly uniform temperature. In summer, the body is cooled by the rapid evaporation of the water in the blood through the pores of the skin; thus, summer and winter, the water in the body serves to keep a uniform temperature throughout. To maintain the supply thus needed, an ordinary person requires from two to three pints of water per day, in summer more than in cold weather. Plants, likewise, must have water to enable them to absorb the necessary substances of food value from the soil, as also, to make the cellulose, starch and sugar, which they store up in their stems, seeds and fruits. 5. Composition of Water.-By volume, water is composed of hydrogen, two parts, and oxygen, one part. This is usually shown experimentally by the Hoffman apparatus (Fig. 4). R is a reservoir into which the water is poured in filling the apparatus. It is necessary to use water slightly acidulated with some acid, as sulphuric, since pure water is not a conductor of electricity. In filling the side tubes, B, B, the stop-cocks are carefully opened, one at a time, and the water allowed to flow in until it barely reaches the stop-cock level. Fig. 4. Electrolysis or Hoffmann apparatus. Not much more should be put into R than will fill the three tubes, for a small amount of water makes a large quantity of gas and this forces the excess back into the reservoir, hence room must be left to receive it. Platinum strips, A, A, serve as electrodes and are connected with the source of current by means of wires sealed in glass tubes passed through the corks, C, C. When everything is ready the current is turned on; bubbles immediately begin to rise from both electrodes, much faster from the cathode than the anode. After a few moments the quantity of gas in each tube may be read from the graduations etched on the tubes, B, B. It will be found. that the quantity of one gas is always double that of the other. To know that the smaller volume is oxygen, hold a splinter with a spark on the end over the tip of the tube and carefully open the stopcock. The pressure of the water in R will force the gas out and ignite the splinter. This characteristic test for oxygen has been mentioned in the preceding chapter. The usual method of testing hydrogen is by lighting it. A burning match or better, a small candle, brought to the tip of the tube containing the larger quantity of gas, will ignite it when the stop cock is cautiously opened. The flame at first will be invisible, or until the glass becomes red-hot; but a piece of paper held to it will be instantly ignited, thus showing the presence of a flame. 6. Explanation of the Experiment. Students frequently ask why the hydrogen goes to the cathode or negative electrode, and the oxygen to the anode. This will be readily understood if it is remembered what was said in the preceding chapter about compounds consisting of a positive and a negative element or group. Since oxygen belongs to what we call the negative group, it would necessarily be attracted to the anode: while hydrogen, being positive, would be attracted to the negative electrode. 7. Proof of Composition by Weight. Since gases are very light substances it is necessary to obtain their weights indirectly in this experiment. In Fig. 5 hydrogen is obtained from any suitable generator, K, which for convenience may well be a Kipp apparatus. In order that the gas may be perfectly dry it is allowed to bubble slowly through a wash bottle, containing concentrated sulphuric acid, which is an excellent dry ing agent in that it absorbs water readily. The tube, T, made of hard glass, contains copper oxide, preferably in what is known as the wire form. Before connecting, this tube with contents is carefully weighed. In the U-tube calcium chloride in small lumps is placed, the tube and contents carefully weighed and connected to the combustion tube as shown. The hydrogen is then turned on, the heat applied, gently at first, until the glass is well warmed, and the operation continued until the contents of C have become red like bright copper. The heat is then turned off, the hydrogen allowed to flow until the tube is cooled enough to handle comfort Fig. 5.-Composition of water by weight. ably when both T and U with contents are again carefully weighed. What has happened is as follows: Hydrogen has the power of taking oxygen away from many oxides when heated strongly. It does so in this case and leaves in the combustion tube mostly pure copper. The loss of weight in this tube, therefore, is the weight of the oxygen used. The hydrogen and oxygen at the temperature present combine to form water, which in the condition of vapor passes over and is absorbed by the calcium chloride in tube U. The gain here, therefore, is the weight of the water produced. Subtracting the weight of the oxygen used from that of the water formed gives the weight of the hydrogen. Allowing for experimental errors which are always possible, it will be found that the average of a large number of experiments carried out thus is always 8 parts of oxygen to 1 of hydrogen. A typical case with data obtained by actual experiment is given below: Copper oxide and tube before heating. .37.23 grams .29.87 Loss, which is oxygen.. 7.36 Calcium chloride and tube, before heating. .25.18 Weight of same, after heating. .33.46 8.28 Gain, which is the water.. Subtracting the oxygen from weight of water 0.92 8. Law of Definite Proportions. It was stated in the preceding chapter that a compound is a substance containing two or more elements united in a fixed and definite proportion by weight. The above experiment illustrates the definition and at the same time shows proof. Out of this truth, which applies to all compounds, grows the "Law of Definite Proportions," which is usually stated thus: When two or more elements unite chemically to form a compound they always do so in the same fixed and definite proportion by weight. Why they must necessarily do this will be taken up at another time in Chapter VI. 9. Hydrates-Water of Combination.-All have seen. various substances in crystalline form, such as rock candy, alum, blue vitriol, or such natural compounds as iron pyrite, silica, called rock crystal, and galena. Crystals of artificial compounds are usually prepared by dissolving the substance in water and allowing the water to evaporate. When this occurs, very often a considerable portion of the water combines with the dissolved |