, ETC.] 



UNDULATORY FORCES. HEAT. 



19 



It must not, however, be assumed that the solid state 

 of bodies can be directly attained by liquids on their 

 arriving at a temperature such as we have indicated in 

 the foregoing skeleton table, although such is true with 

 respect to the conversion of solids to the liquid state. 

 The various forces of nature act together ; and thus water 

 may be cooled much below its point of congelation with- 

 out becoming solid, provided it is kept perfectly still. 

 This interference of apparently heterogeneous and yet 

 correlative forces, may be easily observed on trying the 

 following experiment : 



Experiment 10. Dissolve, in boiling water, as much 

 sulphate of soda (Glauber's salts) as the liquid will take 

 up. This is best performed in a glass flask. Place the 

 vessel and contents on several folds of flannel in a still 

 place, and allow all to attain the temperature of the 

 atmosphere, and the liquid state will be maintained. 

 If, however, a small piece of any angular substance is 

 dropped into the flask, or if its contents are violently 

 shaken, the liquid will instantly pass into the solid 

 state. 



By exposing a glass vessel filled with water to a tem- 



Cture below freezing point, the water will retain its 

 ,d state for some time ; but, on the least agitation, it 

 will be converted into ice. It has been stated, that even 

 the rays of the light of the moon will produce the same 

 effect. We are not in a position to verify this by 

 personal experience, and would suggest, that as there is 

 great difficulty in maintaining a quiescent position, 

 owing to the chances of motion being unwittingly coin- 

 mum~ f ed to a liquid, it is highly possible that an ex- 

 periment of this kind may be vitiated by accidental 

 circumstances. 



Whilst speaking of substances which change their 

 physical- condition at comparatively low temperatures, we 

 may mention that different bodies assume a variety of 

 forms by the peculiar arrangement of their particles 

 during the act of solidification. Thus we have crystal* 

 of snow, the arborescence of ice, and the crystallographic 

 forms peculiar to each body produced at that moment. 

 We shall devote a special article to this subject ; and 

 content ourselves, for the present, by merely mentioning 

 the occurrences as commonly taking place. 



U !..'ii a solid assumes a liquid state at a temperature 

 higher than that of the boiling point of mercury, we are 

 utilised to resort to other means of ascertaining the tem- 

 perature than the employment of the mercurial thermo- 

 meter, because a temperature of 670 Fahrenheit would 

 destroy the instrument. For such purposes pyrometers 

 are employed ; and although much ingenuity has been 

 directed to construct an instrument of this class, on 

 whose indications reliance may be placed, it is to be 

 regretted that no great success has attended such efforts. 

 The earliest form of pyrometer, or fire-measurer, was 

 that of Mr. Wedgwood. It was constructed on the 

 assumption that a piece of clay would contract in propor- 

 tion to the amount of heat to which it wag subjected. 

 The student may be here surprised to learn that a body 

 cnntraett by the application of heat ; but the difficulty is 

 only an apparent one ; for when clay contracts on being 

 heated, the effect is produced solely by the water between 

 its particles being expelled ; hence it forms no exception 

 to the general law of expansion by heat. Owing to the 

 variety of the texture of clay, Mr. Wedgwood's pyrometer 

 affords unreliable results, and has, therefore, been dis- 

 carded. 



The late Dr. Danioll invented an ingenious arrange- 

 ment, wherein the expansion of a Kir of platina was made 

 to register high temperatures. The bar is enclosed in a 

 cylinder of black-lead crucible ware ; and a piece of 

 porcelain, which fits tightly into the cylinder, and 

 touches one end of the platina bar, indicates, by the re- 

 moval from its first position, the extent of expansion 

 which the platina has undergone during the application 

 of heat. By a mechanical arrangement, the expansion 

 i.i registered in degrees corresponding with the scale of 

 I -iheit 



r arrangements have boon made, intended to facili- 

 tate the registration of high temperatures ; but, in every 



case, the groat difficulty which has to be contended with 

 is, that the intense heat to which the indicating sub- 

 stance has to be exposed, almost always vitiates the 

 result obtained. 



As it is usual, in scientific works, to give a table of the 

 melting point of various solids, we follow that course : 

 however, the reader must bear in mind, that some of 

 the figures in the following table have been calculated 

 only approximately, and are liable to the sources of error 

 which have been pointed out. 



Melting point, according to the degrees of the scale of 



Fahrenheit, of the metals. 

 Potassium .... 136 F. 



Sodium 194 



Tin 442 



Bismuth ...'.. 476 



Lead 694 



Zinc 700 



lli-.i-s - 1869 



Silver 1280 



Copper 2548 



Gold 2590 



Cast-iron 3479 



The above may be fused by means of furnaces of 

 various kinds ; but platina, and many other bodies, re- 

 quire the powerful heat of the oxyhydrogen blow-pipe, 

 or of the voltaic battery, to reduce them to the liquid 

 state. 



Platina and wrought-iron, however, seem to enter into 

 a partial fluid state long below their melting point ; and 

 hence separate masses of either of these metals may be 

 united at a white heat, so as to form one piece. It is 

 impossible to state definitively the reason of this property 

 being confined to two substances ; and it is the more re- 

 markable because their physical and chemical diameter 

 are wholly distinct, and, in many instances, the very anti- 

 podes to each other. The process, as we previously ex- 

 plained, is termed "welding." 



Advantage is taken of the power of heat to fuse bodies, 

 for the purpose of forming what are termed "alloys." 

 Thus brass ia formed by fusing together copper and zinc. 

 The consideration of this will be duly observed in our 

 chemical section ; and to that we must also defer noticing 

 the effect which heat has in rendering substances soluble 

 in liquids, and of thus facilitating numerous scientific and 

 economic processes. 



A most singular effect is observed when fusion is 

 effected by heat produced by electricity. The combina- 

 tion of these two forces produces the anomalous result of 

 sometimes fusing a body so suddenly, and of permitting 

 its reconsolidation, as not to allow time for the heat to 

 pass to surrounding bodies ; and hence, during violent 

 thunder-storms, instances have occurred in which metallic 

 articles, such as coins, chains, <fcc. , worn by a person who 

 has been struck, have been completely melted ; whilst the 

 linen or cotton on which such have rested has been en- 

 tirely uninjured. We have frequently noticed that a 

 powerful Leyden discharge will superficially melt a wire, 

 and yet leave a piece of paper, on which the metal rested, 

 perfectly unburnt. No adequate reason has been as yet 

 assigned for such occurrences. 



Having mentioned the lead- 

 ing methods of measuring in- 

 tense heat by means of the 

 expansion of solids, we must 

 not omit to describe a very 

 ingenious instrument, invented 

 by M. Breguet, of Paris, for 

 ascertaining minute changes of 

 temperature. It is constructed 

 on the principle that two me- 

 tals, expanding unequally on 

 receiving increments of heat, 

 will assume a curved form 

 a fact which we illustrated by 

 Experiment 7.* 

 The annexed engraving illustrates one of these inter- 

 esting arrangements. The spiral is formed of a ribbon 



Ante, p. 10. 



