FLUORESCENCE. 67 



I have been a little anticipating what was to come, 

 namely, that these phenomena are not confined to fluids or 

 clear solids, but that they can be seen in every case. I 

 have shown you that in a spectrum if you separate out the 

 rays from one another by prismatic refraction you can see 

 the phenomena in the invisible rays. But there is another 

 mode of separating light into two portions of which one 

 is allowed to pass, which is easier in practice, and which 

 exhibits some of these phenomena very beautifully that 

 is by analysing it by absorption. For instance, I have here 

 a very deep blue glass which cuts out most of the visible 

 light, but it admits the violet and certain invisible rays 

 beyond very copiously. It is a cobalt blue glass, and here 

 is a yellow glass. I will analyse the light, not by a prism, 

 but simply by absorption. This jar at present contains 

 nothing but water. Now I will put the blue glass on to 

 the electric lamp, and I have here a solut: on obtained from 

 the bark of the horse chestnut, a little of which I will drop 

 into the jar. If you make a decoction of the bark, which 

 contains a good deal of tannin, the solution soon becomes 

 brown. It contains however two crystal lizable substances, 

 called esculin and fraxin, which can be obtained chemically 

 pure. The alkaline solutions of these bodies have this 

 power in a high degree. You can get rid of the tannin by 

 making a decoction, and when cold adding some persalt of 

 iron or salt of alumina, precipitating by ammonia, and 

 filtering, and you then get this beautiful solution, which 

 will keep very fairly. I will pour some of this into the 

 water, and as it sinks down it forms a beautiful blue cloud. 

 If I hold the blue glass so as to intercept the incident rays 

 hardly any diminution will be perceived, while the yellow 

 glass cuts off most of the rays by which the effect is pro- 

 duced ; but if I put the yellow glass between the jar and your 

 eyes, a great deal of the light is transmitted The general 

 effect is shown very well by means of these glasses. Here 

 also are some jars containing some fluorescin and other fluids 

 which can be tried in the same way. A coloured medium will 

 absorb in a different manner the rays that fall on the fluid arid 

 the rays coming from it. That leads to one method of obser- 

 vation, which does not require the apparatus which I have 

 hitherto supposed, but is exceedingly simple and at the same 

 time very effective. You do not even require sunlight, 



F 2 



