58 



THE POPULAR EDUCATOR. 



silver is only 6. One important fact which strikes us hero is 

 that their conducting- power for electricity seems to correspond 

 closely with that for heat. 



We shall now understand the reason why metals and other 

 bodies feel cold to the touch. They are good conductors, and 

 therefore carry away rapidly the heat from the part of the body 

 with which they are in contact ; bad conductors, on the other 

 hand, only rob us of a small amount. As a general rule, all 

 organic substances, and those which are loose in texture, are 

 bad conductors, hence these are selected as the materials for our 

 clothing. A great mistake is often made in supposing that 

 clothing actually imparts heat ; the real fact is, that it merely 

 keeps in the heat which is produced in the system. The human 

 body is considerably above the surrounding 1 air in temperature, 

 being kept so by that portion of our food which is burnt in the 

 system. This heat would be very rapidly dissipated, and im- 

 parted to the air and surrounding objects, did not our garments 

 intervene and, by their non-conducting power, prevent its escape. 

 A further illustration that this is really the case is seen in the 

 fact that ice-carts are carefully covered over with blankets, 

 certainly not with the intention of keeping the ice warm, but 

 for the sake of keeping out the warmth of the air, which would 

 rapidly melt it. 



Air is a bad conductor ; hence loose bodies, such as sawdust, 

 shavings, or tow, which enclose a largo amount of air in their 

 interstices, are frequently employed to exclude cold. Water, 

 likewise, is a very bad conductor. This at first seems unlikely, 

 when we remember how quickly a quantity of water may be 

 brought to the boiling point ; but we shall soon S"e that this is 

 not heated by conduction, but by convection. To prove this, 

 we may take a large jar of water, and, having placed a deli- 

 cate thermometer at the bottom, set light to a tin saucer of spirit 

 floating on the top. A large amount of heat will thus be pro- 

 duced, and the saucer will soon become intensely hot ; the ther- 

 mometer at the bottom, however, will remain unaffected for a 

 long time. A simpler way of proving thia fact is shown in Fig. 

 23. A test-tube is filled with ice-cold water, some fragments of 

 ice being kept at the bottom. A spirit-lamp may then be applied 

 to the upper part, and the water there will boil for a long time 

 before the ice at the bottom is melted. This would not be the 

 case if the water could conduct the heat. 



That important invention, the safety-lamp, depends for its 

 action on the conducting power of the metals. The lamp is 

 entirely surrounded by a shade composed of wire gauze, which 

 virtually consists of a large number of very short tubes, placed 

 side by side. As the flame attempts to pass through these, its 

 heat is conducted away, and it is no longer able to ignite the 

 explosive gases outside. 



The mode in which the metal conducts the heat away will be 

 easily seen by taking a cylinder, one end of which, A (Fig. 24), is 

 composed of wood, while the other end is of metal. If now we 

 wind a piece of paper round this, and hold it in the flame of a 

 spirit-lamp, the paper over the wooden part will be charred, 

 while that over the other end will merely be smoked, the metal 

 underneath having conducted away the heat before it had time 

 to scorch the paper. This also explains how a bullet may be 

 melted in a piece of writing-paper. The paper must be wrapped 

 smoothly round it, and the flame allowed to play only on the 

 part in contact with the lead. The metal will, of course, burn 

 through the paper as soon as it is melted, but up to this time 

 jhe heat is all employed in melting the lead, and is thus kept 

 away from the paper. 



If we take a few flakes of solid carbonic acid, procured as de- 

 scribed in our last lesson, and place them on the hand, they will 

 not feel as cold as we should expect. The reason of this is 

 that they become slowly converted into gas, which keeps them 

 from absolute contact with the hand. If a little ether be 

 mixed with them, and the mixture dropped on the heat, intense 

 cold will be produced, and all the effects of a severe burn will 

 be experienced. If a lump of frozen mercury be taken up in 

 the finger, exactly the same result will be produced. We see, 

 then, that an intensely cold substance burns as an intensely hot 

 one does. K a quantity of mercury be frozen, with a wire in it 

 to serve as a handle, it may be lifted like a solid mass. Now 

 dip it into a vessel of water, and in a short time it will begin to 

 melt, drops of it falling to the bottom of the vessel. These, as 

 they fall, will absorb so much heat as to freeze tubes in the 

 water, down which the mercury will run. 



LESSONS IN ENGLISH. LI. 



VERBS EEVIEW VEBB-PABSING. 



THE three root-forms of the English verb may pass into a great 

 variety of forms, as appears from the following combinations of 

 the verb to teach : 



1. With the Infinitive Mood. I do teach, I shall teach, I will teach, 

 I may teach, I cau teach, I must teach, let me teach. 



2. With the Participle Present. I am teaching, I shall be teaching, I 

 will be teaching, I may be teaching, I can be teaching, I must be 

 teaching, let me be teaching. I have been teaching, I shall have beec 

 teaching, I will have been teaching, I may have been teaching, I cau 

 have been teaching, I must have been teaching, let me have been 

 teaching. 



3. With the Participle Past. I am taught, I shall be taught, I will be 

 taught, I may be taught, I can be taught, I must be taught, let me 

 be taught. I have taught, I shall havo taught, I will have taught, I 

 may have taught, I cau have taught, I must have taught, let me have 

 taught. I have been taught, I shall have been taught, I may have 

 been taught, I might have been taught, I can have been taught, I must 

 have been taught, let me have been taught. 



Mark that when two or more of what are called auxiliary 

 verbs are combined with a participle, usually the first expresses 

 the manner and the second the time ; the first only admits of 

 variation in itself (inflection), as, I might have loved, thou mightst 

 have loved. 



The forms just given havo to be multiplied first by the per- 

 sons three singular, three plural ; secondly, by the tenses 

 present and past ; thirdly, by if and other conjunctions giving 

 rise to the dependent and the elliptical constructions. Then 

 there are the affirmative, the negative, and the interrogative 

 forms ; as well as the interrogative-negative. Besides this 

 there is the uncontracted and the contracted form, as well as 

 the solemn or the scriptural form : for example : 



1. Forms multiplied by the Persons. I am teaching, thou art 

 teaching, he is teaching, we are teaching, you are teaching, they are 

 teaching. 



2. Forms multiplied by the Tenses. I teach, I taught, I was teaching. 



3. Forms multiplied by IF, etc. If I teach, if I taught, if I am 

 teaching, if I was teaching, etc. 



4. Affirmative, Negative, Interrogative, and Interrogative-negative Forms, 

 etc. I teach ; I do not teach ; do I teach ? do I not teach ? 



5. Contracted and Uncontracted Forms. They dou't teach, they do 

 not teach ; I don't teach, I do not teach. 



6. Sci-iptu?-ol Forma. He teacheth, he loveth, he instructeth, he 

 guideth. 



A few facts respecting the verb remain to be set forth. 



The most general division of verbs is that which exhibits 

 them as personal and impersonal. 



Personal verbs are such as take the ordinary persons J, thou, 

 he, etc. Impersonal verbs take only the third person of the 

 neuter gender, namely, it; for example, it rains, it snows, it 

 hails, it thunders. It has been proposed to call these verbs 

 unipersonal (having one person), on the ground that impersonal 

 signifies that which has no person. I do not know that the 

 proposed change is worth adopting. Strictly speaking, it is 

 not a person, inasmuch as a thing is not a person. It is more 

 important to remark that in these impersonal verbs the action 

 of the verb is represented in the most abstract form of which it 

 is possible next to the infinitive. Thus the noun snow passes 

 into an indefinite verbal shape in to snow, and to snow becomes 

 a little less indefinite in the form of it snows. Impersonal verbs 

 mostly refer to atmospheric changes, and are used less exten- 

 sively in English than in most other languages. 



Verbs may be divided into primitive and derivative. The 

 primitive are intransitive, the derivative are transitive. The 

 change is effected for the most part by operating on the vowel : 

 for example : 



Primitive. Eise, lie, sit, fall, drink. 

 Derivative. Eaise, lay, set, fell, drench. 



I have already given instances of verbs derived from nouns 

 by a change in the accent or pronunciation. Another class of 

 verbs is formed from nouns by hardening the final consonant ; 



