It is usually understood that rennets are calves' stomachs salted and dried, or otherwise prepared; but it is not so certain that all the rennets in market are of this kind. The stomachs of the young of all milk-eating animals may be used for curding milk. We are not so sure but that among "Bavarian" rennets we get the stomachs of the young of every animal known under the sun. They are of all sizes and all degrees of strength, but are generally liked by those who use them. They are cured by tying the two ends, and blowing the rennets up, like bladders. A better way, we think, is to rub them well with pure dairy salt, stretch them on a hoop or crotched stick, and hang them in a cool dry place. Some simply fill them with salt, tie them, and hang them up to dry. A great objection to this is, that the salt is likely to draw moisture from the atmosphere, and in wet weather the rennets are liable to drip and thus lose strength. Salting rennets down in a barrel, as we do meat, is considered objectionable--for what reason, we know not. The writer had excellent "luck," one season, with rennets preserved in this way. In whatever way preserved, rennets should, by all means, be kept cool. Heat is found to be very injurious, while cold--even freezing and thawing--appears advantageous. Possibly because the freezing and thawing loosen the fiber and set the rennet spores free. AGE AN ADVANTAGE. No rennets less than a year old should be used, if it can possibly be avoided. The old rennets, other things being equal, are stronger and make a firmer curd than new ones. Any one who has experimented with both will always aim to have a supply of good old rennets on hand. SAVING RENNETS.