To capture the acetic acid lime is added to the mixture, and since there is a natural affinity between them, the acetic acid and lime combine into a solid which remains behind when the whole mass is suitably heated. What comes over in the form of vapour is a mixture of water, acetone and wood spirit. The former is enticed away by the use of quicklime, while the other two are separated by the process of fractional distillation already referred to.

Now let me ask you to form another little picture, either in your mind or with paper and pencil. Imagine two methyl radicles, each, let me remind you, a carbon atom with three hydrogen atoms hooked on and one spare hook. Also imagine one atom of oxygen with its two hooks outstretched like two arms,

and just link one radicle on to each. Then you have the picture of methyl ether. All the ethers are formed by taking two of the paraffin radicles and linking them together by means of the two hooks of an oxygen atom. The ether which is so largely used in hospitals for wounded soldiers is ethyl ether, consisting of two ethyl radicles joined by oxygen. How it is made we will come to in a moment, but as you see already it is a close relative of alcohol.

Now from methyl ether take away the central oxygen and in its place put carbon. This atom will have two hooks to spare which it can employ to hold on to the two hooks of the oxygen. The result is a molecule of acetone.

This is used as a solvent in a similar manner to alcohol for many purposes, and there was a great demand for it no doubt during the war.

One interesting use of acetone is in connection with the gas acetylene. Of great use both for lighting and also in conjunction with oxygen for welding and cutting metals, this gas suffers from the disadvantage that it cannot be compressed into cylinders and carried about as oxygen can. It can, however, be dissolved in acetone. The cylinders in which it is carried are therefore filled with coke saturated with acetone and then when the acetylene is pressed in it dissolves, coming out of solution again as soon as the pressure is released. In this dissolved condition it is quite safe to carry about.

For a moment let us turn back to the commencement of the chapter to the subject of methane. When mixed with chlorine, it will be remembered, one

hydrogen atom gave place to a chlorine atom. If the process be repeated another hydrogen atom will be displaced in the same way, while a further repetition will result in the removal of a third, when there will be a carbon atom in the centre with three chlorine and one hydrogen hooked on to it. With that picture in your mind's eye you will be contemplating the molecule of that wonderful and beneficent substance, chloroform. When we think of the numberless operations which have been carried out by the surgeons in the course of this last war we realize a little how great is the total sum of pain and suffering which has been saved through the agency of this substance, this simple neat little arrangement of five tiny atoms.

Now that again is obtained in manufacture from alcohol. Alcohol, bleaching powder and water are mixed and then distilled, by which of course is meant that the mixture is evaporated by heat and the vapour collected and cooled back into liquid again. The liquid so obtained is chloroform.

Hardly less important than this, in our military hospitals, is ether, to which reference has already been made. It, too, is manufactured from alcohol. The alcohol, together with sulphuric acid, is placed in a still and heated, the vapour given off being led to another vessel and there condensed. The liquid thus obtained is ether and so long as the supply of fresh alcohol is kept up the production of ether goes on continuously.