THE 



POPULAR EDUCATOR. 



LESSONS IN CHEMISTRY. I. 



INTRODUCTION ATTRACTION OP GRAVITY FORCE Of 

 COHESION FORCE OF AFFINITY. 



THE object of Chemistry is to ascertain the nature and proper- 

 ties of the substances of which the crust of our planet is com- 

 posed. Of late years, the curiosity of the chemist has pene- 

 trated beyond the tangible, and by the aid of the " spectrum 

 analysis," of which in due time we shall treat, a new chapter 

 has been added to the science on " Stellar Chemistry," which 

 gives some insight into the composition of the great centre of 

 our solar system, and even of the distant stars. In pursuing 

 his investigations, the chemist submits the bodies under his 

 consideration to experiment, he operates upon them with various 

 forces heat, electricity, etc. brings them within the action 

 of re-agents, watches their behaviour in all circumstances, and 

 never predicts a result, but determines all by experiment ; 

 hence chemistry is purely an experimental science. 



Seeing that wo have to do with bodies, let us in this lesson 

 dwell upon the forces which act upon " substance,' ' and which 

 oppose or assist the chemist in his research. 



What is body ? " That which haa weight ' ' is, perhaps, the 

 least objectionable definition. Gases, although they are so 

 intangible, and unlike anything solid, are yet bodies ; they have 

 weight. The weight of air on every square inch is 14'67 Ibs., and 

 when set in motion it becomes wind, which sways the trees, carries 

 before it clouds of dust, or sweeping in the hurricane it devas- 

 tates a country, which it could not do if the air were imponder- 

 able. There are, however, existences present in the world 

 which have no weight. Caloric, which produces the phenomenon 

 of heat ; electricity ; ether, whose waves cause the sensation of 

 light, and the different forces of attraction these, not being 

 "bodies," do not strictly come within the range of Chemistry: 

 they rather belong to the domain of the physicist ; but it will 

 be necessary to speak of them, seeing they take such a promi- 

 nent part in the decomposition and combination of bodies. 



The forces of attraction, by which the particles of bodies are 

 bound together, are the attraction of gravity, the attraction of 

 cohesion, the attraction of adhesion, and the force of aflinity. 



The attraction of gravity is that mysterious power by which 

 the Creator has linked to each other the suns and worlds which 

 occupy space ; for he has ordained that all matter should exert 

 an attractive force on all bodies in its neighbourhood. This 

 force varies with the mass of the bodies and their distances 

 from each other. If a stone be dropped over the edge of a high 

 perpendicular cliff into the sea beneath, it will strike the rock 

 before it reaches the water, because the cliff attracts the stone 

 and draws it towards it. If, however, the stone bo carried away 

 from the cliff, the attractive force decreases. The power which 

 made the stone fall was " gravity," that is, the attraction which 

 the earth has for the stone ; the force of that attraction we call 

 its " weight." That this force decreases with the distance the 

 atone is taken above the earth, is proved by the fact that the 

 stone would weigh less on the top of a high mountain than in 

 the valley beneath. Of course, to test this fact a spring balance 

 must be used. 



The force of cohesion, which has more claim upon our attention, 

 differs from "gravity" chiefly in this, that "gravity" acts upon 

 bodies at a distance, whereas the force of cohesion only begins 

 to operate when the particles of matter are brought into the 

 closest contact. 



It is due to this force that the bodies possess solidity, and it 

 would seem that in liquids " cohesion " was very weak, and had 

 no existence at all in gases. 



If I file a piece of iron, the teeth of the file separate small 

 pieces of the metal from the rest ; that is, I have applied a force 

 97 N.E. 



which has overcome the power of cohesion, and therefore certain 

 particles have been wrenched from their neighbour!. Now I 

 may collect the filings and submit them to the greatest 

 pressure I can exert, but I cannot bring them back into their 

 solid state ; no pressure which we at present poaoeas seems to 

 be capable of bringing the particles sufficiently near to each 

 other to allow the force of cohesion to come into play. 



But although particles of bodies are bound thud closely 

 together, yet in no body do they seem to be in actual contact, 

 for all solids are porous. Two hundred years ago this WM 

 proved in the case of gold by the "Florentine Experiment;" and 

 if gold, which is almost the densest of metals, can be shown to 

 be porous, we may well believe it of the rest. The "Florentine 

 Experiment " is so celebrated that it demands recital. The 

 question was raised concerning the compressibility of water, and 

 it was determined to try the experiment in the following 

 manner : A hollow sphere of gold was filled with that liquid ; 

 and seeing that a sphere is that solid which possesses the maxi- 

 mum capacity, any alteration in its shape would therefore lessen 

 the quantity of water it could contain. The gold globe wa 

 accordingly slightly flattened, and the water oozed through the 

 gold, appearing as dew on the outside. The Florentines, there- 

 fore, declared the water was not compressible a conclusion 

 they had no right to draw unless they could have collected the 

 dew, and found that it exactly filled the space by which the 

 pressure had diminished the capacity of the hollow sphere. 

 Water has been proved to be slightly compressible, and the 

 only use of the " Florentine Experiment " is to assert that gold 

 is porous. 



This truth, that the particles of bodies, in spite of the great 

 force of cohesion, are not in actual contact, may be inferred 

 from the fact that all bodies contract when cooled, which they 

 could not do if their particles were already in contact. Thus it 

 would appear that the particles of bodies are under two forces 

 one attracting, the other repelling them, for there must be 

 some force which keeps the particles apart ; and that the state 

 of the substance, whether it be solid, liquid, or gaseous, will 

 depend upon the ratio which these two forces bear to each 

 other. In the solid state the molecular attraction, or cohesion, 

 is by far the stronger. In the liquid condition the repelling 

 power almost balances the attractive ; in a gas it entirely super- 

 sedes it, and the atoms are solely under the influence of mole- 

 cular repulsion. When the temperature of a body is raised, 

 this molecular repulsion is always increased, each atom being 

 repelled from its neighbour. The body expands, and at last the 

 cohesion is so nearly overcome that the solid becomes a liquid. 

 If the temperature still increase, the atoms are still furthei 

 repelled, until they cease to have any attraction for each other, 

 and the body becomes a gas. Molecular repulsion is so closely 

 allied to caloric, the one is so intimately dependent upon the 

 other that they have been thought to be one and the same 

 thing. 



That the physical condition of a body entirely depends upon 

 heat may be shown in almost all bodies. Ice becomes, when 

 heated, water then steam. Put a small piece of zinc in the 

 flame of a blow-pipe : it first becomes red-hot, then melts, and 

 finally goes away in vapour, which burns with a bright white 

 flame, into the oxide of zinc. There is the strongest evidence 

 that all bodies are capable of assuming these three states. 

 Solids may be gases under certain circumstances ; and gases, by 

 sufficiently reducing their temperature, may become, first liquids, 

 then solids. In future lessons we shall find many examples of 

 this interesting fact. 



Adhesion is a force which binds two bodies together by mean* 

 of some adhesive substance, such as gum, glue, etc. 



The force of affinity. This is eminently a chemical force. 



