The basis of many of these artificial media is _broth_. This is made from good lean beef, free from fat and gristle, which is finely minced up and extracted in sterilised water (one pound of lean beef to every 1000 cc. of water). It is then filtered and sterilised. It will be understood that such an extract is acid. To provide _peptone beef-broth_, ten grains of peptone and five grains of common salt are added to every litre of acid beef-broth. It is rendered slightly alkaline by the addition of sodium carbonate, and is filtered and sterilised. _Glycerine-broth_ indicates that 6 to 8 per cent. of glycerine has been added after filtration, _glucose-broth_ 1 or 2 per cent. of grape-sugar. This latter is used for anaërobic organisms. The use of broth as a culture medium is of great value. It is undoubtedly our best fluid medium, and in it may not only be kept pure cultures of bacteria which it is desired to retain for a length of time, but in it also emulsions and mixtures may be placed preparatory to further operations. _Gelatine_ is broth solidified by the addition of 100 grams of best French gelatine to the litre. Its advantage is twofold: it is transparent, and it allows manifestation of the power of liquefaction. When we speak of a liquefying organism we mean a germ having the power of producing a peptonising ferment which can at the temperature of the room break down solid gelatine into a liquid. _Grape-sugar gelatine_ is made like grape-sugar broth. _Agar_ was introduced as a medium which would not melt at 25° C., like gelatine, but remain solid at blood-heat (37·5° C.; 98·5° F.). It is a seaweed generally obtained in dried strips from the Japanese market. Ten to fifteen grams are added to every litre of peptone-broth. Filtration is slow and often difficult, and the result not as transparent as desirable. The former difficulty is avoided by filtering in the Koch's steamer or with a hot-water filter, the latter by the addition of the white of an egg. Glycerine and grape-sugar may be added as elsewhere. _Blood agar_ is ordinary agar with fresh sterile blood smeared over its surface. _Blood serum_ is drawn from a jar of coagulated horse-blood, in which the serum has risen to the top. This is collected in sterilised tubes and coagulated in a special apparatus (the serum inspissator). _Potato_ is prepared by scraping ordinary potatoes, washing in corrosive sublimate, and sterilising. They may then be cut into various shapes convenient for cultivation. Upon any of these forms of solid media the characteristic growth of the organism can be observed. Of the nutrient elements required, nitrogen is obtained from albumens and proteids, carbon from milk-sugar, cane-sugar, or the splitting up of proteids; salts (particularly phosphates and salts of potassium) are readily obtainable from those incorporated in the media; and the water which is required is obtainable from the moisture of the media.