124 



NATURE 



\_May 30, 1878 



This is an apparatus for burning a mixture of ordinary 

 gas and air ; the gas should be supplied through the 

 smaller tube inserted into the lower end of the larger one, 

 as shown in Fig, 3. The gas should be lit by holding a 



Fig. 3. — Bunsen burner, ordinary form, a, ext«rior tube ; s, holes to 

 admit the air. 



match some 4 inches above the upper orifice of the wide 

 tube. One end of the platinum wire may be fused into 



l!i!liirill|!!!l!|' 



Fig. 4. — Improvised Bunsen burner 



the piece of glass tubing, and the other twisted] into a 

 loop, fine enough to hold some common salt in the flame. 

 A piece of coke or charcoal soaked in salt and water will 



Fig. 5.- 



^ do almost 



-Method of inserting platinum wire and salt into the flame. 



as well. This tube may be supported by a''piece 



of wood after the fashion of Fig. 5. The Bunsen burner 

 will give us a very hot bluish flame, into which the loop of 

 platinum containing common salt, sodic chloride, may be 

 inserted, as shown in the accompanying woodcut. We shall 

 get a brilliant yellow flame, which is worth notice on its own 

 account, and if the artist is performing these experiments 

 in his studio, let him look at some of his choicest pic- 

 tures by means of this light, before he goes any further. 

 He will be considerably surprised at their appearance, 

 and I hope he will set himself to think about the cause of 

 it — a point on which there will be a good deal to be said 

 in the sequel. It will be better, however, to get at the 

 physics in the first instance. To do this, put the impro- 

 yised Bunsen burner and the platinum wire with the salt 

 on it in front of the slit, and look at the slit through the 

 prism ; it will be found that there is only a yellow image 

 of the slit visible. If the things have been nicely arranged, 

 the appearance of the spectrum will be so entirely changed 

 that a beginner will be apt to fancy that something has 

 gone wrong. Nothing has gone wrong however ; we 

 have simply passed from the spectrum of polychromatic 

 to that of monochromatic light — from white light to 

 coloured light. 



These experiments touching the giving out of light can 

 be easily and cheaply varied by burning green and red 

 fire in front of the slit; the effect of these differently- 

 coloured lights on a picture is also very striking. 



The next thing we have to do then is to represent the 

 action of an absorbing body,— to study the action 

 of our theoretical screen — the action of bodies when 

 they absorb light, and therefore transform the origmal 

 colour which that light possessed. Liquids will form our 

 most convenient screen to illustrate this, and they can be 

 placed in a " cell " like that shown in the accom- 

 panying woodcut. 



Fig. 6. — Common^fonn of cell to hold solutions. 



It will not be necessary to buy such an apparatus ; two 

 squares of glass, with a piece of india-rubber tubing 

 between them, bent in the shape of a U ; the glasses be- 

 ing kept in contact with the tubing by two india-rubber 

 bands, form a cell which is wonderfully tight, and will 

 serve our present purpose. This cell should be placed 

 in front of the slit. 



A little potassic permanganate added to the water in 

 the cell, will act as a screen, and cut off the yellow 

 part of the spectrum, and the adjacent regions of the 

 orange and green. Solutions of blood or magenta will 

 give also very definite indications of absorption, and if we 

 have one of those handy little pocket spectroscopes, which 

 now, I am glad to think, are becoming common, the 

 absorption of the light of a candle by the blood in the 

 lobe of a friend's ear, or in the interval between two closed 

 fingers can be well seen by placing it between the slit and 

 the light. 



Let us now sum up as tersely as may be the con- 

 clusions at which we have so far arrived. 



I. White light analysed by a prism gives us a con- 

 tinuous spectrum. 



