Elements of chemistry : including the recent discoveries and doctrines of the science / by Edward Turner.

  • Turner, Edward, 1798-1837.
Date:
1835
Catalogue details

Licence: Public Domain Mark

Credit: Elements of chemistry : including the recent discoveries and doctrines of the science / by Edward Turner. 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.)

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    the thermometer sinks by exposure to cold, the enamel likewise retreats to- wards the bulb, owing to its adhesion to the spirit; but, on expanding, the spirit passes readily beyond the enamel, leaving it at the extreme point to which it had been conveyed by the previous contraction. For registering the highest temperature, a common mercurial thermome- ter of the same form as the preceding is employed, having a small cylin- drical piece of black enamel at the surface of the mercury. When the mercury expands, the enamel is pushed forward; and as the stem of the thermometer is placed horizontally, it does not recede when the mercury contracts, but remains at the spot to which it had been conveyed by the previous dilatation. The enamel is easily restored to the surface of the mercury by slight percussion while the bulb is inclined downwards ; but this should be performed with care, lest the enamel, in falling abruptly, should interrupt the continuity of the mercurial column, and interfere with the in- dications of the instrument. The risk of this accident is lessened by putting some pure naphtha in the tube beyond the mercury, and its presence is like- wise of use in preventing the oxidation of the mercury. The above descrip- tion applies to an improvement on Dr. Rutherford's thermometer, made by Mr. Adie of Edinburgh. Though the thermometer is one of the most valuable instruments of phi- losophical research, it must be confessed that the sum of information which it conveys is very small. It does indeed point out a difference in the tem- perature of two or more substances with great nicety ; but it does not indi- cate how much heat any body contains. It does not follow, because the ther- mometer stands at the same elevation in any two bodies, that they contain equal quantities of heat; nor is it right to infer that the warmer possesses more of this principle than the colder. The thermometer gives the same kind of information which may be discovered, though less accurately, by the feelings; it recognizes in bodies that state of caloric alone, which affects the senses with an impression of heat or cold,—the condition expressed by the word temperature. All we learn by this instrument is, whether the temperature of one body is greater or less than that of another; and if there is a difference, it is expressed numerically, namely, by the degrees of the thermometer. But it must be remembered that these degrees are parts of an arbitrary scale, selected for convenience, without any reference whatever to the actual quantity of heat present in bodies. Very little reflection will evince the propriety of these remarks. If two glasses of unequal size be filled with water just taken from the same spring, the thermometer will stand in each at the same height, though their quan- tities of heat are certainly unequal. This observation naturally suggests the inquiry, whether different kinds of substances, whose temperatures as esti- mated by the thermometer are the same, contain equal quantities of heat,— if, for example, a pound of iron contains as much heat as a pound of water or mercury. The foregoing remark shows that equality of temperature is not necessarily connected with equality in quantity of heat; and the in- ference lias been amply confirmed by experiment. If equal quantities of water are mixed together, one portion being at 11)0° and the other at 50°, %he temperature of the mixture will be the arithmetical mean or 75°; that is, the 25 degrees lost by the warm water will exactly suffice to heat the cold water by the same number of degrees. It is hence inferred, that equal weights or measures of water of the same temperature contain equal quan- tities of heat; and the same is found to be true of other bodies. But if equal weights or equal bulks of different substances are employed, the result will be different. Thus if a pint of mercury at 100° be mixed with a pint of water at 40°, the mixture will have a temperature of 60°, so that the 40 degrees lost by the former, heated the latter by 20 degrees only; and when, reversing the experi- ment, the water is at 100° and the mercury at 40°, the mixture will be at 80°, the 20 degrees lost by the former causing a rise of 10 in the latter. The fact is still more strikingly displayed by substituting equal weights for mea- sures, For instance, on mixing a pound of mercury at 160° with a pound