4th, Mercury accommodates itself more speedily to the temperature of the medium in which it is placed, than any other fluid, gases excepted. The theory of the heating and cooling of bodies is a very curious one, and till lately was very imperfect ly understood. From its great importance, a short outline of it may here find a place. Equal quanti ties of heat thrown into different bodies do not ne cessarily produce the same temperatures in each. An equal bulk of mercury having a temperature of 100' and of water at 40° will not produce a mixture having a temperature at a mean of these, or 70°, as equal volumes of water would have done; the re sulting temperature will only be 90°, showing that the 40° of heat lqst by the mercury when transfer red to the water had only the power of raising it 20° of temperature. Speaking generally, therefore, mercury is capable of receiving only half as much heat as water without being raised to a higher tem perature, or to use the language of modern chemis try, the specific caloric of water being 1, that of mercury by equal volumes is 0.5.§ Now it is easy to see, that as the quantity of heat required to raise mercury to any temperature is less than is re quisite for any other fluid, the acquisition of tem perature will, ca'leris paribus, be more quickly ac complished. Add to this that mercury combines the rapid conducting power of a metal with conduc tion by locomotion, which is peculiar to the fluid state, and which, communicating the temperature through all its parts, facilitates the change of con dition. Dr. Trail found, (Nicholson's Journal, xii. p. 137.) by a series of experiments, in which the real conducting power of substances was alone in dicated, that mercury conducted a definite portion of heat in 15" for which alcohol required 10' 45" making the conducting power as 43 to 1. From these circumstances a mercurial thermometer ac quires much sooner the temperature of any me dium than one of alcohol; a fact which, though long since ascertained, was a subject of great sur prise and perplexity to philosophers. The deter mination of specific heats being one of great nicety, experimenters vary very much in their results, but probably we are not far wrong if we consider alcohol to possess double that of mercury. As a satisfactory proof of the superiority of mercury in this respect, we shall quote a specimen of Martine's experiments on the subject, which were among the first to throw light on the subject, taken from his little volume of Essays on Heat,l a work which at any period would have done honour to its author, and for the time at which it was written, certainly one of uncommon merit. The following experi ment was made by placing two thermometers, one containing 31 ounces of water, the other near ly 48 of mercury (or equal volumes) before a large fire.
Here we observe that the mercury reached the point of 92a° in 24', for which the water required 52', whence to find the relative capacity we say as 52 : 1 : 24 :0.46, or exactly the most modern de duction. :Mercury being thus compared to water as a standard, the following experiment by the same author gives the relative value of spirit of wine: Ifere the spirit of wine acquired the temperature of 88° in 32', for which the water required 44'. Hence 44 : 1 : : 32 :0.73 the relative specific heat. We think, therefore, that the fourth recommenda tion of mercury is proved, as acquiring any tem perature in a shorter time than any other thermo metric fluid, for of course the rapidity of cooling depends on the very same circumstances as that of heating.
5. The fifth and last qualification of mercury is the perfect uniformity of its nature and properties, wherever it is procured, requiring only the opera tion of a careful distillation to render it fit for all thermometric purposes, and different specimens perfectly comparable. In this it differs remarkably from alcohol, which it is impossible to procure everywhere of precisely the same degree of purity, which greatly affects the extent and regularity of its expansions.
We have entered the more fully into these pro perties of mercury, as the consideration of them forms the whole basis of the first theory of the thermometer; we therefore see how great a benefit Reaumur and Fahrenheit conferred upon science by its introduction. The instruments made by the latter in Amsterdam were extremely neat, vying in portability and elegance with the productions of modern artists. Sometimes the scale was composed of paper, and being fixed in the form of a scroll round the tube, the whole was inclosed in a glass case, hermetically sealed. A very elegant instru ment of this description we have seen, made by Prius, Fahrenheit's partner, at Amsterdam, which once belonged to the late Baron Masercs. Dr. Patrick Wilson of Glasgow used to make very beautiful thermometers on the same construction. Sometimes, particularly abroad, the scale was formed by a roll of paper inclosed within the glass tube BC, Plate DXXIV. Fig. 3, attached by a so lid bend to the top of the thermometer at C. This plan is now out of use in this country; but we have often seen it employed by artists in Italy. Mercu rial thermometers are more usually made in this country in a style resembling Fig. 4, the scale be ing of ivory, and sometimes the bulb and stem pro ject below from the scale for the purpose of greater delicacy; the bulb too is then generally made elon gated like the instrument of Fahrenheit.
The scale adopted by this philosopher was rather an arbitrary one, and the motive of its adoption we have not been able quite satisfactorily to discover. Several of the older writers whom we have consult ed, such as Boerhaave,t simply affirm that at the zero of the scale, the mercury was conceived to be divided into 11124 parts, an expansion of one of which indicated a degree, but how that arbitrary number was discovered is not indicated. Muschen broek, on the other hand, leads us to believe from the details of the construction of the thermometer in his Essais de Physique, § 948, that the highest point of the scale was placed at the boiling point of mercury, and the space between that and the zero of the scale divided into 600 degrees. This, how ever, seems hardly probable, since Fahrenheit al ways employed boiling water in preference to boil ing mercury in graduating his instruments. Were we to propose any theory, we should conceive the most probable one, that this philosopher, in con formity with the principle so much attended to, previous to his time, attempted to make his de grees equivalent to „4„ of the bulk of mercury at some fixed point, which, in fact, they are so ex tremely near, as to render the coincidence, if an accidental one, extremely remarkable. Some, we believe, suppose that he selected the number 64 as divisible by continued bisections for the interval in degrees between the freezing point and blood heat, but this appears a less probable hypothesis.