Species of bacteria for comparative observation-experiments upon the action of chemical or physical agents must be not only the same species, but cultured under the same conditions, and treated by the agent in the same manner, otherwise the results cannot be compared upon a common platform, or with any hope of arriving at exactly the same conclusions. Sir George Buchanan laid down, in 1884, a very simple and suitable standard of what true disinfection meant, viz., the _destruction of the most stable known infective matter_. Such a test is high and difficult to attain unto; nevertheless, it is the only satisfactory one. Obviously many substances which are useful antiseptics in practical life would fall far short of such a standard, yet for true and complete disinfection such an ideal is the only adequate one. Quite recently three or four workers at Leipzig[101] have drawn up simple directions, the adoption of which would considerably assist in securing a common standard for disinfectant research. They are as follows: 1. In all comparative observations it is imperative that _molecularly equivalent_ quantities of the reagents should be employed. 2. The bacteria serving as test objects should have equal power of resistance. 3. The numbers of bacteria used in comparative observation should be approximately equal. 4. The disinfecting solution must be always used at the same temperature in comparative experiments. 5. The bacteria must be brought into contact with the disinfectant with as little as possible of the nutrient material carried over. (This obviously will depend upon the object of the research.) 6. After having been exposed to the disinfectant for a fixed time, they should be freed from it as far as possible. 7. They should then be returned in equal numbers to the respective culture medium most favourable to the development of each, and kept at the same, preferably the optimum, temperature for their growth. 8. The number of surviving bacteria capable of giving rise to colonies in solid media must be estimated after the lapse of equal periods of time. We may now turn from general principles to mention shortly some of the commoner methods and substances adopted to secure efficient disinfection. They are all divisible, according to Sir George Buchanan's standard, into two groups: 1. Heat in various forms; 2. Chemical bodies in various forms. It should at the outset be understood that we desire in practical disinfection to inhibit or kill micro-organisms without injury to, or destruction of, the substance harbouring the germs for the time being. If this latter is of no moment, as in rags or carcasses, burning is the simplest and most thorough treatment. But with mattresses and beddings, bedclothes and garments, as well as with the human body, it is obvious that something short of burning is required.