138 



THE POPULAR EDUCATOR. 



The absorbing power, likewise, is just equal to the radiating 

 -power ; they appear to be, in fact, almost synonymous terms. 

 The difference caused in the absorbing power by the nature of 

 the surface may easily be shown. Let the beam of an electric 

 lamp fall upon the clean bulb of a differential thermometer ; 

 the rays, as they have already passed through the glass lenses 

 and through a stratum of air, will impart no heat to the ther- 

 mometer, which will remain unaffected. If now we lay a little 

 lamp-black on the bulb, the heat will at once be absorbed, and 



the bubble driven to the other limb. 



Many common practices can easily be explained by noticing 

 the different absorbing and radiating powers of various sub- 

 stances. A dish-cover or metal teapot is kept as bright as 

 possible, so as to prevent the escape of the heat by radiation ; 

 a black earthenware teapot, on the other hand, has a dull and 

 dark surface, so that it may be placed on the hob and absorb the 

 heat. So, too, if a kettle is to heat quickly, the part exposed 

 to the fire should be covered with fur and soot, to absorb the 

 heat; the other part should be bright, to prevent its radiation. 

 These things, like many similar ones, were known and put in 

 practice long before their true causes were known, but science 

 now shows us how to account for them. 



The laws of radiation likewise account for the deposition of 

 dew at night. The air is then cooler than the surface of the 

 earth, and the latter accordingly radiates its heat into space. 

 Those bodies, therefore, which are the best radiators become 

 cool most rapidly, and therefore condense the vapour which 

 exists in the air. Plants radiate freely, and hence become 

 coated with dew, while a smooth road remains almost dry. 



Clouds, to a great extent, prevent this radiation, and hence 

 the dew will be most plentiful en a clear and cloudless night. 

 A very thin layer of calico or matting is likewise sufficient to 

 retard radiation, and for this reason gardeners often place a 



covering of this kind over delicate plants to protect them from 

 injury by the cold. When the temperature of the ground is 

 very low the dew freezes as it is deposited, and constitutes hoar- 

 frost. 



When experimenting with radiant heat, we find, as already 

 referred to, that substances differ greatly in the amount of heat 

 they allow to pass through them. This may be easily tested by 



the arrangement shown in Fig. 30. A screen, B, is interposed 

 between the source of heat, A, and the thermo-electric pile, D ; 



; all stray rays are thus cut off, and only those which pass in a 

 straight line through the aperture C can reach the pile. Under 

 c is a small shelf, on which we can place the bodies to be tested. 

 \ glass cell filled with bisulphide of carbon and placed there, 

 will allow about 63 per cent, of the rays to pass, while if filled 

 with water, it will only allow 11 per cent. ; other liquids may 

 .also be tried. Among solids, rock-salt is the substance most 

 transparent to heat, as it allows about 92 per cent, of the rays 

 to pass. With most substances the amount of heat transmitted 



varies with the nature of the source of heat, the heat from a 

 -'oil of incandescent platinum wire, for instance, having a 



greater penetrating power than that from a plate of copper 

 *tt 750. 



We must, however, leave the student to pursue these in- 

 quiries further, the object of these lessons to give a general 

 insight into the main facts of the science of Heat having been 

 .accomplished. 



LESSONS IN FRENCH. LXXXV. 

 133. OBSERVATIONS ON THE BEPETITION OF ADVERBS. 



1.) THE adverbs of comparison, plus, moins, must be re- 

 jeated before every adjective which they modify : 



et He is less idle and obMnale than 

 his brother. 



II est moins paresseux 

 moins obstine que sou frere. 



(2.) These adverbs, and the adverbs of quantity, need not be 

 repeated before every noun ; but the preposition de, which 

 must always come between peu, trop, beaucoup, tant, plus, 

 moins, autant, assez, combien, and a noun or an adjective, 

 used substantively, must be repeated in every case : 



II n'y aurait pas tant de peine 

 et de uiisere dans ce niourle . . . 

 Ce librairo a beaucoup de bons 



ret de mauvais ouvrages dans son 

 iinagasin. 



There would not be so much trouble 

 and misery in the world . . . 



This bookseller Tins many good and 

 bad books in his e. tablishment. 



(3.) The adverbs mieux, better ; pis, worse, must not be con- 

 founded with the adjectives meilleur and pire. See note 

 15, (8). 



134. ADVERBS OF NEGATION. 



(1.) The negation is composed of ne placed before the verb, 

 and pas or point, jamais, etc., after it in the simple tenses. 

 The second negative comes between the auxiliary and the verb, 

 in the compound tenses : - 



Heacen does not regulate things 

 according to our wishes. 



Le ciel sur uos sonhaits ne 

 regie pas les choses. 



CORNEILLE. 



Home n'attache point le grade 

 a la noblesse. CORNEILLE. 



L'estime est le vrai principe 

 de la consideration, qui n'est pas 

 toujours attache'e aux dignit^s. 



FOXTENELLE. 



Les rois ne sont point proteges 

 par les lois. CHENIER. 



II n'a jnmais dit cela. 

 Je ne chante guere. 



Rome does not by any means con- 

 fine offices to the nobility. 



Esteem is the true principle of 

 consideration, which is not always 

 attached to offices. 



Kings are by no -means protected 

 by laws. 



He has never said that. 

 I do not sing much. 



It will be seen in the above examples, that the negative point 

 is stronger than pas. The meaning of these two words, which 

 are in fact substantives used adverbially, and express the signi- 

 fication of the negative ne, will sufficiently explain this : 



N'allez pas means n'allez un pas, do not go or move one pace 

 or step. N'allez point means n'allez un point, do not go or 

 move a point or dot. 



(2.) When the verb is in the present or in the past of the 

 infinitive, the two negatives may be put together before the 

 verb, or the verb between them : 



In order not to go out. 

 For never having told a He, 



Pour ne pas sortir ; or pour ne 

 sortir pas. 



Pour ne jamais avoir menti ; 

 or pour n" avoir jamais menti. 



The first of these two constructions is the most generally used. 



(3.) The second negative may be suppressed after the verbs 

 pouvoir, oser, savoir, and cesser : 



Non, deesse ; je ne puis souffrir 

 qu'uu de leurs vaisseaux fasse 

 naufrage. FENELON. 



Dans son appartement, elle 

 n'osait reiitrer. VOLTAIRE. 



Qui vit ha'i de tous, ne saurait 

 longtemps vivre. C' VNEILLE. 



La libert^ ne o tee d'etre 

 aimable. CORNEILLE. 



No, goddess; I cannot suffer that 

 a, single one of their vessels perish. 



She durst not re-enter her apart- 

 ment. 



He who I ices hated bj all, cannot 

 exist long. 



Liberty cannot ce^se to be worthy 

 of love. 



(4.) Pas or point is not used when the verb is modified by 

 another negative word, such as jamais, guere, nul, nullement, 

 aucun, personne, ni ; by ne followed by que, meaning only ; 

 and by ne followed by plus, meaning no more : 



L'ambition, seigneur, n'aguere 

 de limites. BOURSAULT. 



Nul n'est heureux, s'il ue jouit 

 de sa propre estime. 



J. J. EOUSSEAU. 



Personn.9 n'aime a recevoir de 

 couseils. DE SKGUK. 



Un me'chant ne salt jamais 

 pardouner. NOEL. 



Ambition, my lord, has scarcely 

 any limits. 



No one is happy, unless he can 

 esteem himself. 



No one likes to receive advice. 



A wicked man never Tenam how to 

 forgive. 



(5.) With two verbs, the adverbs of negation are placed with 

 the one they are intended to modify : 



Je ne puis pas y aller. 



je puis ne pas y aller. 



II n'OSC pas le dire. 



ll ose ne pas le dire. 



I cannot go there. 

 I may not go there. 

 He does not dare to say si. 

 He is impudent enough not to 

 say so 



(6.) Ne used idiomatically. 



The negative ne is used without any negative sense after the 

 conjunctions a moins que, unless ; de peur que, de crainte que, 



for fear that : 



Unless you speak to him. 

 For /ear, or Zest you might oe 

 deceived. 



A moins que vous ne lui parliez. 

 De peur qu'on ne vous trompe. 



L'ACADEMIE. 



(7.) Ne is used in the same manner after autre, different ; 

 autrement, othenvise ; plus, moins, mieux, forming a compari- 



* 



