This section is from the book "The Steward's Handbook And Guide To Party Catering", by Jessup Whitehead. Also available from Amazon: Larousse Gastronomique.
Liquid used to mix with fresh milk to change it to sweet curd, whereof cheese is made. This sweet curd is useful also in some kinds of dessert, as curds with clotted cream, and, drained as if for cheese, it is mixed with eggs, etc., in various sorts of cheese cakes. Rennet is obtained by soaking the inside lining of a calf's stomach (cleansed and prepared) in water. " Get a calf's bag from the butcher's - in some places they keep them already pickled for the purpose. If you can get one pickled, cut it in halves, and put half in about a pint and a half of strong salt and water; let it stand a day or two, then use the rennet as required, taking care to add fresh salt and water in proportion as it is taken out, to keep up the supply. The other half of the calf's bag keep in reserve in the pickle as it comes from the butcher, and as the rennet from the first half becomes too weak, add a portion of the second half to keep up the strength. About a tablespoon-ful to two quarts of milk is the amount required; let stand in a warm corner for 2 or 3 hours.