UNDULATORY FORCES. LIG II T. [ootODM or lunnuL 01 



the eye, but which are .vspectively discovered l>y placing 

 ]KI|HT coated with chloride of silver \-\ "iid I!. and a 

 thermometer beyond A, when these peculiar properties 

 will become evident. Progressing below the spectrum nt 

 A, and itill further from the red riiya, we shall gradually 

 arrive at another spectrum, wherein the red of the first 

 will merge into the extreme blue of the second ; and 

 hence, after passing the last heating in red rays, these 

 are necessarily succeeded by others of an actinic nature, 

 or capable of producing chemical changes. 



In our section on Photography we shall have to avail 

 ourselves of these principles to a large extent. We 

 must, however, to save space, presume that our readers 

 will at this point fully master the details of our analysis 

 of the spectrum, and thus render continual repetition ou 

 our part unnecessary. 



Before leaving this interesting subject, we recommend 

 the student to try the following experiments, which will 

 tend to impress the truths we have stated on his mind. 



Experiment 7. Prepare a strong solution of sulphate 

 of copper in cold water, and add liquid ammonia to it 

 until all the precipitate is dissolved, and a rich blue colour 

 produced. Pour this into a flat glass vessel, and allow 

 the rays of sunlight to pass through the liquid, and to 

 impinge ou a pieeo of paper coated with chloride of silver. 

 This is made by dipping one surface of a sheet of white 

 paper into a solution of common salt, and, after drying 

 it, the salted side is placed in a solution of nitrate of 

 silver, and subsequently dried. The actinic rays of light 

 pass freely through the solution, and will speedily turn 

 the paper black. 



Experiment 8. Repeat this experiment, but employ a 

 weak solution of bichromate of potass in cold water. 

 The prepared paper will scarcely change at all ; because, 

 in this instance, the liquid permits the luminous and 

 calorific rays to pass, but entirely stops those of ac- 

 tinism. 



Experiment 9. Prepare three solutions one as in 

 Experiment 7 ; another as in the last experiment, so as to 

 be of a rich yellow colour ; and another with the same 

 salt, so as to give a deep orange or red tint ou looking 

 through it. Expose at the back of each of these 

 solutions a small thermometer, so that its bulb may 

 receive the transmitted rays. That behind the blue 

 solution will exhibit no increase of temperature ; and 

 that behind the yellow one will not indicate any extra- 

 ordinary change; while that which receives the rays 

 from the red solution will speedily evidence that heat 

 easily passes, and that to a very much greater extent 

 than through the two other liquids. 



It will thus be perceived, that the three effects of light 

 may be separated from each other ; or, in other words, 

 that which we conventionally call " light" may be divided 

 into three phases, so far as its effects are concerned ; 

 namely, those of actinism, luminosity, and caloric. 



We here append a few instances in which the action of 

 the rays of light effect either physical or chemical changes 

 on some substances ; indicating, at the same time, the 

 rr.iture of the change which is produced. We, however, 

 need scarcely remark, that our list is only illustrative, 

 ami, of course, far from exhausting the almost infinite 

 nnmter of such changes which are constantly occurring 

 in nature, although they are rarely observed by us, be- 

 cause they take place so gradually. We shall omit those 

 which more particularly pertain to photography, because 

 such will come under our consideration when we treat on 

 that subject. 



The Human Skin. Every one is familiar with the fact 

 that the natives of the tropical regions are generally of a 

 colour approaching to black. There are many causes 

 which tend to produce this appearance besides that of 

 light ; and as such pertain rather to physiology, we 

 cannot, of course, enter into their consideration in this 

 place. We shall, however, deal with them in our section 

 under that head, and also under Ethnology. Wo 

 need scarcely remind our readers, that a lengthened 

 exposure to strong sunlight., of inhabitants of tempe- 

 rate regions, has the effect of darkening the colour of 

 the skin. This takes place most readily with persons 



whose complexion is fair. Light, however, has not only 

 a c.,1. Hiring eil'.-ct, luit also must lie regarded as having a 

 decided sanitary inttiifiice on all constitutions. \\ e 

 hence find the ruddy and healthy appearance of out-door 

 labourers, of sailors, and locomotive engineers. Instances 

 are on record of persons who, having acquired deformity 

 and disease through close conhncmeut from daylight, 

 have been speedily restored to health by subsequent 

 iv n iv. d to places where free exposure to the sun was 

 obtained. Our experience of the beneficial effects of 

 a temporary residence at the sea-side, is another illustra- 

 tion of this result ; and such arises not simply from a 

 change of air, but also from the fact, that the rays of the 

 line purer, and more free from loss and absorption 

 of their actinism, when passing through an unclouded 

 atmosphere, than through the smoke of our large cities. 

 \Ve have repeatedly tried experiments, during the same 

 day, in different parts of London and Glasgow, with pho- 

 tographic apparatus ; and have been surprised to find tlio 

 variation of time required to obtain a good image on a 

 sensitive plate, at places respectively free from, or be- 

 clouded with, the smoke of the city. Any one, especially 

 of a fair complexion, may repeat these trials in a rough 

 but interesting manner, by walking for an equal length 

 of time, on two clear days, during the heat of summer on 

 one in the streets of a large city, and subsequently in 

 the open country adjacent to it. In the former instance 

 the skin will be scarcely affected ; whilst, out of town, an 

 exposure to sunlight will often prove extremely painful, 

 from its action on the skin of the face and hands. That 

 such is the effect of light, and not of heat, is evident, 

 inasmuch as the red heating rays are more prevalent 

 under the smoky sky than in a clear one. The former 

 absorbs, whilst the latter permits the free passage of 

 actinic rays ; hence the results to which we refer. Wo 

 may, perhaps, be permitted to adduce an illustration 

 which we afford, individually, of this effect. Whilst ex- 

 perimenting, some years ago, with the substance found in 

 the Russian " infernal machines," we had the misfortune 

 to receive serious injury from the sudden combustion of 

 several pounds' weight of chlorate of potass and sugar ; 

 by whicli the face and hands were severely burnt. 

 Although not the least external evidence exists of this at 

 the present time, an exposure of a few hours to a 

 summer's sun induces extreme pain, wliilst we never find 

 the least inconvenience from the intense heat of a wind 

 or steam-boiler furnace. We have also observed, that the 

 electric light from a large voltaic battery has a similar 

 effect, even although we may not perceive the least 

 sensation of heat. We have named tliis as an interesting 

 physiological fact, and one which we trust that none of 

 our readers may ever be in a position to verify. 



It has been stated by Dr. Edwards, that tadpoles, 

 if entirely deprived of light, are never changed into com- 

 plete frogs, but remain growing in what we may term the 

 tadpole state. Without atlirming the exactness of this 

 statement, wo adduce it as one which is frequently quoted 

 in illustration of the effects of light. 



We have already referred to the fact, that the changes 

 of colour both of the leaves and ilowers of plants, are due 

 solely to the action of the actinic rays. Wo may here 

 add, that some gum-rosins are much acted on by the 

 same agency : amongst them, as a good example, we may 

 name guiacum, which becomes of a green colour when 

 exposed to the light of the sun. 



So long as flowers retain their vital state, the brilliancy 

 and richness of their colour is enhanced l>y light. When, 

 however, their tint is conveyed to fabrics, as in the pro- 

 cess of dyeing, such colours speedily fade on exposure to 

 the solar rays. Pink and rose- red colours, produced from 

 safflower, and the blue colours afforded by litmus, are 

 instances of this kind. Tincture of litmus, which is often 

 employed in thermometers, and which although in a 

 vacuum where no ordinary chemical change could take 

 place, often loses its colour. This, however, is speedily 

 restored by exposing the litmus to tho action of the 

 oxygen in the atmosphere. 



Many chemical compounds change their colour or 

 character from the action of light. Nitric acid, if rery 



