A manual of chemistry : on the basis of Dr. Turner's Elements of chemistry : containing, in a condensed form, all the most important facts and principles of the science designed for a text book in colleges and other seminaries of learning / by John Johnston.

  • John Johnston
Date:
1846
Catalogue details

Licence: Public Domain Mark

Credit: A manual of chemistry : on the basis of Dr. Turner's Elements of chemistry : containing, in a condensed form, all the most important facts and principles of the science designed for a text book in colleges and other seminaries of learning / by John Johnston. Source: Wellcome Collection.

Provider: This material has been provided by the National Library of Medicine (U.S.), through the Medical Heritage Library. The original may be consulted at the National Library of Medicine (U.S.)

    22/488
    found, by this method, to conduct heat better than any other sub- stances; and of the metals, silver is the best conductor; gold comes next; then tin and copper, which are nearly equal; then platinum, iron, and lead. Some experiments were made by Despretz, apparently with great care, on the relative conducting power of the metals and some other substances, and the results are contained in the following table: Gold 1000 Silver 973 Copper 898.2 Platinum 381 Iron 3/4.3 Zinc 363 Tin 303.9 Lead 179.6 Marble 23.6 Porcelain 12.2 Bine Clay 11.4 The substances employed for these experiments were made into prisms of the same form and size. To one extremity a constant source of heat was applied, and the passage of heat along the bar was estimated by small thermometers placed at regular distances, with their bulbs fixed in the substance of the prism. The results, as contained in the table, do not agree entirely with those of Ingenhouz, or of other experimenters, but they may without question be considered as a near ap- proximation to the truth. ] 1. A convenient method to determine the relative conducting power of different substances, is, to have them made into cylinders of equal diameter, and set in a thin piece of wood at sufficient distances from each other, both extremities of each piece projecting from one to two inches from the wood. JI the board be held in a horizontal position, a small piece of phosphorus may be placed upon the upper extremity of each of the substances experi- merited upon, and the lower ends exposed to the same temperature by plunging them in heated oil, or sand that has been previously well mixed after being heated ; and the times that elapse before the ignition of the phosphorus upon the several substances, will indicate with some correct- ness, their relative conducting powers. This principle will explain why a piece of paper wound closely around a cylinder of metal and held in the flame of a lamp is not even scorched (or some time, though, if wood had been used instead of metal, the paper would have taken fire immediately. An experiment of this kind is sometimes performed by holding a handkerchief closely in contact with the bank of a silver watch, and touching it with ignited charcoal. It is said the handker chief cannot be burned in this way; but it would be more correct to say, it cannot be done without difficulty. 12. An ingenious plan was adopted by Count Rumford for ascertaining the relative conducing poweT of the different materials employed for cloth- ing He enveloped a thermometer in a glass cylinder blown into a ball at iis extremity, and filled the interstices with the substance to be examined. Having heated the apparatus to the same temperature in every instance by immersion in boiling water, he transferred it into meltine ice, and observed carefully the number of seconds which elapsed during the passage of the