After the escape of the air, bubbles of steam, at first very small in size, are formed at the bottom of the vessel, those formed at first are at once cooled from the whole water not being of an equal temperature, and are condensed before they reach the surface: this very rapid and successive condensation of numerous small bubbles gives rise to that peculiar vibration which occasions what is termed the _singing_ of the tea-kettle, and which, as is well known, is indicative of its approach to the boiling point; when the whole water is uniformly heated, this effect no longer occurs, but the bubbles of steam rise to the surface and escape. After having been heated to 212°, the temperature of water no longer rises; it is not possible, under ordinary circumstances, to increase the temperature in the slightest degree, for all the extra heat that is given to boiling water merely produces an increased quantity of steam, by which it is carried off, without affecting the heat of the remaining water. This is a matter of considerable practical importance in cookery; and it should be always borne in mind, that the most gentle simmer is as efficacious in cooking as the most violent boiling, for the degree of heat in both cases is precisely the same, so that after having once raised the water to the boiling point, the most moderate fire is sufficient in ordinary cases to keep it there; by attention to this point, a most enormous saving may often be effected in the consumption of fuel, although this is a consideration that will be more fully entered into in a subsequent article. Thick liquids, which do not readily permit the escape of steam or the rapid motion between the particles of the fluid, may, however, be readily heated at the part exposed to the fire to a much higher degree, whilst those portions not immediately in contact with the heat are much colder; from this cause they are very apt to be charred, and if articles of food, they are totally spoiled. To avoid this effect, recourse may be had to the _bain marie_, which is simply the same contrivance that may be observed in a carpenter’s glue-pot, applied to the preparation of articles of food, being merely an inner vessel to contain the substance to be heated; this is placed in an outer one, the space between the two containing water. On placing this on the fire, it is obvious that the substance in the inner vessel, being heated solely by the boiling water, cannot possibly become burnt; this most useful contrivance is adopted in all first-class kitchens, and is equally indispensable in the chemist’s laboratory; by its aid, soups, gravies, &c., can be kept hot any length of time without risk, preserves made without burning, &c.; the chief precautions required in its use are, that the inner vessel should be thin and formed of metal, so as to allow the rapid transmission of heat from the boiling water, and care should be taken that the outer vessel does not boil dry. One serious disadvantage