This section is from the book "Health", by W. H. Coefield.
The air we breathe, about which I am going to speak to-night, is a material substance. We feel it when it blows upon our faces, and we find it exercises pressure upon objects on the surface of the earth.
If I take a bottle of water, and invert it in a basin of water, something keeps the water up in the bottle, and that is the pressure of the external air upon the surface of the water in the basin, and that water would remain in the bottle if it were thirty feet high, because the pressure of the external air is capable of keeping up a column of water about thirty-three feet high.
' Air is a mixture of gases. What do we mean by a mixture ? When substances are mixed together the weight of the mixture is of course the sum of the weights of the substances mixed; nothing is lost; and the mixture has properties which are the mean of those of the substances which are mixed. For instance, if I take sugar and sand and mix them together, the weight of the mixture is the weight of the sand and the sugar together, and I get also the mean of the properties of sand and sugar, according to the proportions of sand and sugar that I have taken ; a certain quantity will remain soluble in water, and a certain quantity not. If I make a mixture of alcohol and water, the resulting mixture will have the properties which are the mean of the properties of alcohol and water, for example, the specific gravity will be the mean of the specific gravities of alcohol and water, according to their quantities; that is what is meant by a mixture.
Certain gases exist in the air in the condition of a mixture, so that the properties of the air are the mean properties of those gases in the proportions in which they are mixed. I insist upon this, because a mixture is a very different thing from a chemical compound, and I want you to understand that the air is not a chemical compound.
In chemical compounds the same rule holds as to weight; the weight of a compound is the sum of the weights of the substances combined.
The difference between a mixture and a compound is that when substances are mixed we get the mean of their properties as a result, but you can never predict the properties of a compound from the properties of the substances combined, unless you have learnt previously what is going to be the result. Let us take an illustration.-Suppose I were to put out the gas and then turn the tap on, the gas would escape from the pipe and mix with the air of the room, and the resulting mixture would have properties which would be the mean of the properties of the air in the room, and the gas, according to their proportions. Now, suppose I applied a light to that mixture, you all know that it would explode, and the result would be the production of substances perfectly different from either the air or the gas, substances of which you could not predict the production, if you did not know beforehand the result of the experiment. The substances produced when gas burns in the air are entirely different from either the gas or the air.
To show this more clearly, suppose I take a solution in water of a salt of silver, and a solution of common salt, in two different tubes ; these are two clear colourless liquids. Now, unless you knew beforehand what the result of mixing them would be, it would be impossible to predict it, for if I pour one into the other a white opaque substance, insoluble in water, is produced, which is a compound of one of the substances contained in common salt with the silver ; again, if I take a solution of corrosive sublimate, which is a salt of mercury, and a solution of iodide of potassium, two colourless liquids, and mix them, a salmon-coloured insoluble substance is produced which you could not have expected if you had not learnt what the result would be.
Nitric oxide is a colourless gas which is insoluble in water ; this is capable of combining with one of the constituents of common air, and when they are put together the result is a brownish-red gas, which is soluble in water.
These experiments will show the difference between a mixture and a chemical compound.
There is another important thing to be remembered, and that is, that when two substances are mixed no heat is given out, but when two substances are combined to form a chemical compound heat is produced.
If I take cold water and mix it with brandy it will not make hot grog, but if I take water which is quite cold and another liquid called oil of vitriol also cold, which has the property not only of mixing with the water but of combining with it, and pour them together, they combine, and heat is given out to such an extent that the vessel in which they are cannot be held in the hand because of the heat.
Another thing is that whereas you can mix substances, such as milk and water, in any proportions whatever, substances will only combine in certain definite proportions, and if when two substances are put together to combine, there is too much of one, the surplus remains uncombined, so that they only combine in certain definite proportions.
Air may be made by taking the substances which we know it to contain and merely mixing them together in the proportions in which they are contained in air, and that mere mixture has all the properties of air. Air then, is a mixture of gases. What is a gas ?
The substances that are around us are commonly divided into three kinds-solids, liquids, and gases.
A solid body has a certain amount of rigidity, retains its shape and size, unless it be broken or bent It occupies a certain space.
A liquid is a body which takes the shape of the vessel that contains it, and it occupies a certain space in the vessel.
A gas is a body, any quantity of which, however small, will fill any space, however large, within practicable limits.
A bottle full of any gas would fill a room as well as it fills the bottle. A gas will fill a space although that space is already filled with other gases, and that is what we mean when we say that one gas is to another as a vacuum. Whatever gases there are in a space, when you put another there that fills the space all the same.