HKAT. 



ni:i;i:r.\v I.ANCUAOE. 



MO 



IIMHU re supposed to have been chemically pure, and this is a point 

 of great importance since the presence of small quantities of alloy 

 greatly impain the conducting power. The praeenoe of carbon in 

 iron, exerU a similar effect. Taking silver at 1000, and bar iron at 436, 

 steel was found to be 397, and out iron 359. Mercury in the above 

 Uble u stated too high ; the result being probably influenced by 

 conrectire current* which were not counteracted. 



It has been shown by De Senannont (' Ann. de Chimie,' 3, Serie 

 xxi. xxiL) that the conducting power of homogeneous solids, and of 

 crystals which belong to the regular system, is uniform in every 

 direction, yet in crystals which do not belong to a regular system the 

 conducting power varies in different directions with respect to the optic 

 axis. For example, if a slice of quartz be cut parallel to the axis of 

 the prism, and another slice cut at right angles to that axis, and a 

 silver wire be passed through the centre of each plate so that the other 

 cods may be heated in a flame, a film of bees' wax smeared over each 

 section will be melted differently when the wire is heated. In the 

 plate cut across the axis, the wax will be melted in the form of a circle 

 with the wire in the centre ; while on the other plate the wax v. ill )* 

 melted in the form of an ellipse, the long axis of which coincides with 

 the optic axis of the crystal, thereby showing the superior conducting 

 power in this direction than in the one at right angles to it. De la 

 Rive and De Candolle long since showed that wood conducts heat much 

 better with the grain than across it, and Tyndall (' Phil. Trans.' 1853) 

 has proved that heat passes rather more rapidly in a direction from the 

 external surface towards the centre than in a direction parallel with 

 the ligneous rings, the greatest conducting power coinciding with the 

 direction of greatest porosity and readiest cleavage. 



Heat is reflected from different substances very unequally. The 

 reflective power U greatest in polished metals, but they differ greatly 

 among themselves. Out of 100 rays, according to Melloni, silver 

 reflects 90, bright lead 60, and glass only 10 : the remainder are 

 absorbed. It has been estimated that the sun's rays, in traversing a 

 column of air =6000 feet high, lone one-fifth of their heat by absorp- 

 tion. But Melloni was the first to make known the remarkable fact 

 that the amount of heat absorbed by the same body varies with the 

 source of the heat, with the exception of lamp black, which appears to 

 absorb all the rays which fall upon it from whatever source. Melloni 

 used u sources of heat the naked flame of an oil lamp, a platinum 

 wire heated to redness in the flame of a spirit lamp, a sheet of copper 

 heated to between 700 and 800 in a current of hot air rising from a 

 lamp beneath it ; and a copper canister filled with boiling water. The 

 ball of the thermoscope covered with lamp black, may evidently lw 

 placed at such a distance from each of these sources of heat that the 

 liquid shall stand in each case at the same point, so as to indicate the 

 same temperature. Noting these distances and covering the ball of the 

 thermoacope with another substance instead of lamp black, the 

 instrument will apparently receive different proportions of heat, 

 although placed at the distances at which when coated with lamp black 

 the heat appeared to be equal. For example, if the absorption of lamp 

 black from each source of heat be = 100, the thermoscope coated with 

 white lead at the same distance from the naked flame as before 

 indicated only 53. The following table will show the variations in 

 this respect, or the relative absorbabilities of different kinds of heat. 



Absorbing Surface. 



Lamp black . 

 White lead 



Hinglaw 

 Indian ink . 

 Shell Uo 

 PolUhfd metal 



Naked 



Flame. 



100 



81 



52 



96 



43 



14 



Incandescent 



Platinum. 



100 



M 



M 



95 



47 



18- i 



Copper nt 



7SO'F. 



100 



84 

 87 

 70 

 IS 



Copper .it 

 j,J. F . 



100 

 100 



91 



89 



71 



IS 



Melloni also discovered remarkable differences in the power of certain 

 bodies to transmit heat through them. Those which were transparent 

 to heat he termed diathermanoia or diathermic (from 8m through and 

 tipfu hot) while those which do not allow heat to pass are termed 

 nthrrmatvnu or adiat/iermic. Bodies that are transparent to light are 

 by no means equally so to radiant heat. There is only one known 

 solid that approaches to perfect diathermacy, and that is rock salt. 

 Colourless gases are diathermanoua in the highest degree; but all 

 liquids hitherto examined have considerable absorptive action on the 

 thermic rays. A fuller notice of this subject will, however, be given 

 under RADIATION. For other important divisions of our subject we 

 refer to LATENT HAT ; SPBCIFIC HIAT; STEAM; DEW; EVAPORA- 

 TION; BOILIXQ or LIQUIDS; EBULLITION; FREEZING; CBTOPHORCS; 

 and some other kindred subjects. 



Respecting the chemical agency of heat we give no detail^ seeing 

 that no chemical oi>eration can be performed without some disturbance 

 of temperature or some alteration in the latent and specific heats of 

 bodies. We cannot dissolve any salt in water without a disturbance 

 of temperature, or combine or decompose substances, without similar 

 and often very energetic manifestations of beat. 



We must however just refer to what is called the mechanical theory 

 of heat, which has of late years excited revived attention in conse- 

 quence of the experiments of Joule ('Phil. Trans,' 1860) on the definite 

 amount of heat developed by friction. The mathematical theory of 

 heat started by Caroot was favourable to Joule's result, and has been 



revived by Clauslus, Rankine, W. Thomson, and others. The reader 

 interested in the subject will find an excellent resuml of the theory of 

 the mechanical action of heat or thertno-dynamics by Professor 

 Rankine in Nichol's 'Cyclopedia of the Physical Sciences.' The 

 principle sought to be established by this theory is as follow* -.that 

 in all cases where mechanical effect is produced by heat, a quan 

 heat is used up proportional to the mechanical effect produce i 

 conversely that the same quantity of he it can be again generated by 

 the expenditure of just so much mechanical effect. Thus it appears 

 from Joule's experiments that the actual quantity of heat -I. 

 by friction, depends simply on the amount of force expended, ami nt 

 on the nature of the substances nibbed together. \Vln N 

 example, is agitated by means of a horizontal brass paddlr -win-. 1 

 to revolve by the descent of a known weight, the tempera- 

 l>"mi<l of water is raised one degree Fahr., by the expenditure of an 

 amount of force, sufficient to raise 772 Ibs. to the height of one foot. 

 So also when cast iron U rubbed against iron, the force required to 

 raise 1 Ib. of water 1 Fahr. is about 775 Ibs., and by the agitat 

 mercury with an iron paddle wheel 774 Ibs. These results are the 

 means of a large number of experiments, and the conclusion drawn 

 from them is, that the quantity of heat, capable of raising the 

 temperature of 1 Ib. of water (between 55* and 60) by 1 Kulir., 

 requires for ita evolution the expenditure of a mechanical force, 

 to the raising of 772 Ibs. one foot. This is the mechanical equivalent 

 of a unit of heat, and is known as Joule's equivalent. Expressed in 

 terms of the French metrical system the heat capable of raining one 

 gramme of water 1 C. is equivalent to a force which would lift i 

 grammes through a height of 1 metre. But not only heat and < 

 power, but all other kinds of pbyxical energy, such as chemical action, 

 electricity, and magnetism, can be experimentally proved to be con- 

 vertible and equivalent ; that in, any one of these kinds of force may by 

 ita expenditure be made the means of developing any other in o-rt.iin 

 definite proportions. 



There are certain reservations to be made in using Biich w> 

 heat and temperature, but our remarks on this subject had bettor be 

 deferred until we come to speak of the method of graduating t'- 

 meters. See THERMOMETER. 



HEBREW LANGUACfE forms a branch of that extensive family 

 of languages known by the name of Semitic ; a name which is derived 

 from the real or supposed descent of the people who speak these 

 languages from Shem the eon of Noah. The Semitic languages may 

 be divided into three branches : the Arabic, to which the Ethiopic 

 is closely allied ; the Aramiean, consisting of two dialects the Baby- 

 lonian or East Aramaean (sometimes but erroneously called Chaldee), 

 and the Syriac or West Aramaean; and the Hebrew, to which the 

 Phoenician and Punic are closely related. Of these languages the 

 Arabic is the most copious, and the Aramican the poorest and least 

 developed ; the Hebrew holds an intermediate rank between these, 

 being more perfect than the Aramaean, and inferior to the Arabic. 



The Hebrew language derived its name from the Hebrews. wlm il:itc 

 their origin from Abraham, who is called 'the Hebrew' '"cym 

 in Gen. xiv. 13. The etymology of this word is doubtful. Ace 

 to the Jews it is derived from Eber (~135'> an ancestor of Abraham 

 (Gen. x. 24, 25 ; xi. 15) ; but Gesenius and many other critics maintain 

 that Eber cannot be regarded as a historical person, and that his name 

 has been invented in the same manner as the names of Ion, Doras, 

 -Eolus, Ac , by the Greeks, to account for the origin of the people. 

 It has been supposed with some probability that the name of ' Hebrew ' 

 was originally applied to designate all the Semitic nations west of 

 the Euphrates, which appear to have emigrated from Mesopotamia. 

 According to this etymology, the word ' Hebrew' is derived from the 

 root "137. ' to pass over.' Tim appears to have been the opinion of 

 the translators of the Septuagint, who render Gen. xiv. 13, ' Abram the 

 Hebrew,' by 'A&paji rf riparri, that is, ' Abram the passer-over.' All 

 the descendants of Abraham were, according to this view, originally 

 called Hebrews ; and the name was only restricted afterwards to the 

 inhabitants of Palestine. (See Ewald, ' Hebrew Grammar,' 3 ; ami 

 Gesenius, 'Hebrew Lexicon,' under .^O3M This name in never 

 applied to the language of the Hebrews in the Old Testament ; in 

 Isaiah xix. 18, it is called the language of Canaan (]3??3 Dpii?) ; and 

 in Isaiah xxxvi. 11, 2 Kings xviii. 26, 2 Chron. xxiii. 18, and \el>. 

 xiii. l!4, the Judaic or Jewish language (JTTirP)' The language 

 spoken in Palestine in the time of Christ is frequently called Hebrew 

 ('EfyMuff-rt) in the New Testament (John v. 2; xix. 13; Acts xxi. 40; 

 xxii. 2; xxvi. 14); by which the Aramaean is probably intended. In 

 the writings of the Rabbinical Jews the Hebrew is generally called the 



' holy language' (SEJ'T'lp 7 1 ? 1 ?'' 



The Hebrew language appears to have been formed in Palestine by a 

 union of the ancient Aramaean, which was brought by the Aurahamitea 

 from Mesopotamia, with the Phoenician or Canaanitish, the language of 

 the original inhabitants of the country. That the Phoenician and 

 Hebrew languages were very closely allied is evident from the Phoe- 

 nician names of persons and places, and from the specimens of the 

 I'hn nii-un language which we possess in coins and inscriptions. 

 (Bochart, 'Geographia Sacra," b. ii. cc. 1-7; Bellermann, ' Handbuch 

 der BibL Lit.,' vol. i. sect. 56 ; Bellermann, ' Versuch einer erkliirung 



