The conditions most favourable for the exclusive development of these organisms have been learnt by long practice; but should these conditions at any time fail to be observed, some other ferment, incapable of producing the particular kind of ripening wished for, may intrude itself. The chamber is then said to be “sick,” and has sometimes to be temporarily abandoned. Special members of the yeast and mould families are also largely concerned in the ripening of certain cheeses, and their action is very similar to that mentioned above. Roquefort and Pontgibaud cheeses, for example, are ripened by _Penicillium glaucum_, or, in other words, bread mould. These cheeses are kept as near as possible to 32° F., not because so low a temperature is most favourable to the development of the mould, but because other ferments, and especially such as give rise to putrefaction (vibrios), are thereby checked. From the low temperature and unsuitable soil the ripening is apt to proceed so slowly that it is customary to expedite the fermentation by a liberal inoculation of mouldy bread, and by piercing holes to enable the plant to penetrate inwards. In Gruyère cheese are found long cells constricted in the middle like an elongated figure of 8. These cells multiply by splitting in two at the constricted part, forming two individuals. A gelatinous layer surrounds each cell when young, and also divides and envelopes the new individuals. This, however, disappears with age, leaving the cell naked. The action of this organism is to resolve any milk sugar that may be present into alcohol, acetic acid, and carbonic acid, and as this latter is a gas, it forms a number of small bubbles in the cheese. Gruyère is a cooked cheese; for in order to hasten the elimination of the whey, and enable the curd to be pressed in the mould as quickly as possible, the curd is heated slowly, and with constant stirring, to about 120° F. This requires considerable care, for if the heating be too rapid, the grains formed are large, and in the press flatten out and adhere to one another, and so clothe the cheese with an impenetrable layer, through which the whey is unable to escape. On the other hand, an undercooked curd is liable to retain an excess of whey; and the evil of this is that too much gas is given off by the fermentation of the sugar, and consequently, instead of bubbles, long channels appear in the cheese, which depreciate the value of the product. Again, as the ferment is killed at a temperature very near 120° F. (varying a little with the acidity or alkalinity of the curd), it is very possible to destroy it by overheating, and then the cheese becomes dry, is difficult to mature, and is said to be “dead.” Under any circumstances the ripening of Gruyère cheese is a very slow process.