Two hundred pounds of tar produce, upon an average, fifty-three pounds of essential oil. If the coal-tar is wanted to be converted into pitch, without obtaining the oil which it is capable of furnishing, the evaporation of it may be performed in a common boiler; but as it is extremely liable to boil over, the greatest precaution is necessary in conducting the evaporation. A boiler constructed on the following plan is very convenient for the conversion of coal-tar into pitch. The contrivance consists in adding a spout, or rim, to the common boiler, into which the tar spreads itself as it rises, and by this means becomes cooled, and the boiling over is checked. [Illustration: _Kettle for boiling Tar._] 1000lb. of coal-tar produce, upon an average, from 460 to 480lb. of pitch. A subsequent fusion, with a gentle heat, converts the coal-pitch into a substance possessing all the characters of _asphaltum_. _Ammoniacal Fluid._--The properties of the ammoniacal liquor, which accompanies the tar, and which is deposited in the tar-cistern, has not yet been fully investigated. It is employed already in the manufacture of muriate of ammonia (sal ammoniac). One chaldron of coal affords from 220 to 240lb. of this ammoniacal fluid, which is composed chiefly of sulphate, and carbonate of ammonia.--Such are the products obtainable from coal. However certain the practicability of extending the new lights to the dwelling houses of every town and village is, it cannot be expected that such an event should take place speedily and generally. To eradicate prejudice, and to alter established habits, is a work which nothing but time can effect; because prejudice is the effect of habit, and can seldom be eradicated from the minds of such individuals as consider the ready occurrence of a proposition as a test of its truth. To establish a new philosophical theory has, in every instance, required time sufficient to educate an entire generation of men. The rejection of the Aristotelian philosophy--the adoption of experimental research--the substitution of the doctrine of gravitation instead of that of vortices, and the rejection of phlogiston by modern chemists, are sufficiently illustrative of this assertion. New arts, and new practices, are still more difficult to be introduced. The new art of bleaching need merely be mentioned to prove this assertion. The new grammar--the new rudiments of science--the new stile--or the new instrument, however superior to the old in simplicity, facility, and truth, must be less valuable to the ordinary teacher or artisan, whose memory is familiarized with the precepts of the latter, and whose only ambition is to earn his subsistence with the least possible exertion.