Supposing now, when the meat is first put into water, the water is made to boil, and while boiling a piece of cotton wool is put into the mouth of the tube. The tube may be kept in the same room, at the same temperature as the unboiled one, but no signs of decomposition will be found, however long we keep it. The cotton wool prevents it; for we may boil the water with the meat in it, but it would not be long before bacteria and micrococci are present if the wool is not put in the mouth of the test tube. The conclusion you would naturally draw from this simple but very important experiment is that the wool must have some effect upon the air, for we know well that if we keep the air out we can preserve meat from decomposing. That is the principle upon which preserved meats and fruits are prepared. We should at once conclude that the bacteria and micrococci must exist in the air, perhaps not in the state in which we find them in the water, but that their germs or eggs are floating in the atmosphere. How full the air may be of these germs was first shown by Professor Tyndall, when he sent a ray of electric light through a dark chamber, and as if by a magician's wand revealed the multitudinous atomic beings which people the air. It is a beautiful thing to contemplate how one branch of scientific knowledge may assist another; and we would hardly have imagined that the beam of the electric light could thus have been brought in to illumine the path of the surgeon, for it is on the exclusion of these bacteria that it is found the success of some great operation may depend. It is thus easy to understand how great an importance is to be attached to the purity of air in which we live. This is the practical use of the researches to which the art of surgery is so much indebted; and not surgery alone, but all mankind in greater or less degree. Professor Tyndall has gone further than this, and has shown us that on the tops of lofty mountains the air is so pure, so free from organisms, that decomposition is impossible.