PRACTICAL THOUCIITS ON LIGHT. 
227 
in arranging goods and show-glasses in druggists’ shops, to bear in mind this fact, 
and to so place goods (especially any which would readily take fire) that focussed 
rays of light would never fall on them. 
The circumstance caused me to reflect upon the general influence which light 
exerts upon pharmaceutical preparations stored in shops ; and knowing the pre- 
judical effects of light upon chemicals, it is very natural to ask if it cannot to a 
great extent be prevented from doing so much injury. 
In order that the subject may be made as plain as possible, it may not be amiss 
to give a general idea of the nature of light, and how it acts. To be as brief as 
possible, it may be remarked that a ray of light has been found to have three 
distinct properties at the least; namely, the property of giving heat and light, and 
the power of producing chemical action. 
Men of science have demonstrated this for us by passing a ray of light through 
a prism, and then examining the spectrum produced. The spectrum, as first 
produced by Newton, appeared to him to consist of seven primitive colours ; but 
since his time others have represented that there are only three , and that the 
seven are produced by the mixing of the edges of the coloured bands in the 
spectrum. Thus assuming blue, red, and yellow to be the primitive or funda¬ 
mental colours, the other colours of the spectrum are represented to be produced 
by the meeting and overlapping of the bands, blue and yellow, for instance, pro¬ 
ducing green. It has been found that the red ray contains the most heat, the 
yellow ray the most light, and the blue, or violet ray, exercises the greatest 
amount of chemical action. But while the heating effect of the ray of light 
extends beyond the visible part of the spectrum at its red extremity, so also the 
chemical effect, or actinism as it is called, extends beyond the visible part of the 
spectrum at the other or violet end. It has therefore been assumed that the 
heating and chemical effects are due to vibrations which, although they accom¬ 
pany those of light, are nevertheless distinct from them. For our present pur¬ 
pose, however, we may consider that the heating, luminous, and chemical effects 
of light are associated with the coloured rays mentioned, and that the heating 
and actinic rays are those that do the greatest amount of damage to goods. 
Yellow glass completely prevents actinic action, and white opaque glass is the 
best adapted for keeping anything cool. It appears to be quite a mistake to keep 
wines and syrups in dark coloured bottles, for dark colours absorb heat most readily; 
and opaque white glass would be cooler, on account of its reflecting the heat from 
its surface instead of absorbing it. It is quite evident that if the plate-glass in 
our shop windows was tinted with pale yellow, many things might be placed in 
the window without the risk of suffering so much damage as they many times do ; 
and if all our bottles were tinted yellow, many chemicals would keep longer in 
good condition. It would be interesting to know the exact effects produced 
upon different substances by keeping them in bottles of various colours.* In 
some cases, such as in the preservation of salts of silver, the iodides, etc., yellow 
glass would appear to be the best, on account of its preventing chemical action. 
There might be instances where chemical action was not so intense in its effects ; 
but where the heating rays of light do most harm by elevating the temperature 
and giving encouragement to fermentation, as in the case of syrups, or in caus¬ 
ing too much evaporation, as with ethers, in the first instance yellow glass 
bottles would be the best antidote, and in the second, white opaque glass would 
* Some interesting and important information on the effects produced on vegetable 
colours by the different coloured rays of light will be found in a paper by Sir J. Herschel, 
in the Philosophical Transactions for 1842. It appears that the vegetable colours are 
affected by the luminous ravs rather than the chemical ray, and that the rays most effective 
in destroying a particular colour are those which are complementary to it. Thus vegetable 
blues are most affected by the red orange and yellow rays, purples and pinks by the yellow 
and green rays, and orange yellows by the blue rays.—E d. Phakm. Joinin'. 
