Oct. 14, 1880] 



NA TURE 



563 



spectrum by subtracting the shortest lines, and leaving 

 only the long ones. 



On the hypothesis that the elements were truly ele- 

 nientary, the explanation generally given and accepted 

 was that the short lines were produced by a more complex 

 vibration imparted to the " atom " in the region of greatest 

 electrical excitement, and that these vibrations were 

 obliterated, or prevented from arising, by cooling or 

 admixture with dissimilar "atoms." 



Subsequent work, however, has shown ' that of these 

 short lines some are common to two or more spectra. 

 These lines I have called basic. Among the short lines, 

 then, we have some which are basic, and some which are 

 not. 



The different behaviour of these basic lines seemed, 

 therefore, to suggest that tiot all of the sho>-t lines of 

 specti-a were, in reality, true products of higli temperature. 



That some would be thus produced and would therefore 

 be common to two or more spectra we could understand 

 by appealing to Newton's rule: "Causas rerum natu- 

 ralium non plures admitti debere quam qua; ct verje sint 

 at earum phtenomenis explicandis sufficiant," and imagin- 

 ing a higher dissociation. It became, however, necessary 

 to see if the others would also be accounted for. 



Now if not all but only some of the short lines are 

 products of high temperature, we are bound to think that 

 the others are remnants of the spectra of those molecular 

 groupings first to disapjiear on the application of heat. 



At any particular heat-level, then, some of the short 

 lines may be due to the vibrations of molecular groupings 

 produced with difficulty by the temperature employed, 

 while others may represent the fading out of the vibrations 

 of other molecular groupings produced on the first appli- 

 cation of the heat. 



In the line of reasoning which I advanced a year ago,' 

 both these results are anticipated, and are easily explained. 

 Slightly varying Fig. 2 of that paper, we may imagine 

 furnace A to represent the temperature of the jar spark, 

 B that of the Bunsen burner, and C a temperature lower 

 than that of the Bunsen burner (Fig. i). 



1- ic. I.— A. Highest temperature. C. Lowest temperature. 



Then in the light of the paper the lines b and c would be 

 truly produced by the action of the highest temperature, c 

 would be short and might be basic, while of the linei // 

 and OT, j)i would be short and could not be basic, because 

 it is a remnant of the spectrum of a lower temperature. 



So much then by way of explanation ; it is clear that to 

 make this reasoning valid we must show that the spark, 

 or better still the arc, provides us with a summation of 

 the spectra of various molecular groupings into which the 

 solid metal -which lue use as poles is successively broken up 

 by the action of heat. 



We are not limited to solid metals ; we may use their 

 ■salts. In this case it is shown in the paper before referred 

 to 3 that in very many cases the spectrum is one much 

 less rich in lines. 



The experimental work has followed two distinct lines. 

 I shall refer somewhat in detail to the results obtained 

 along each. The first relates to the extraordinary and 

 beautiful phenomena and changes observed in the spectra 



' Proc. R.S., vol. xxviii. p. 159. 2 Prac. R.S., vol. 



3 Phil. Tram., 1873, p. 258. 



. p. 162 



of vapours of the elementary bodies when volatilised at 

 different temperatures in vacuum tubes. Many of the 

 lines thus seen alone and of surpassing brilliancy, are 

 those seen as short and faint in ordinary methods of 

 observation, and the circumstances under which they are 

 seen suggest, if we again apply Newton's rule, that many 

 of them are produced by complex molecules. 



In this case the appeal lies to the phenomena produced 

 when organic bodies are distilled at varying tempera- 

 tures ; the simplest bodies in homologous series are those 

 volatilised at the lowest temperatures ; so that on sub- 

 jecting a mixture of two or more liquids to distillation, at 

 the beginning a large proportion of the more volatile body 

 comes over, and so on. 



The novelty of the method consists in the use of the 

 luminous electric current as an explorer and not as an 

 agent for the supply of the vapours under examination ; 

 that is to say, the vapours are first produced by an exter- 

 nal source of heat, and are then rendered luminous by 

 the passage of the current. The length and bore of the 

 tube therefore control the phenomena to a certain extent. 



A form of apparatus which I have found to answer 

 very well is shown in the accompanying woodcut (Fig. 2). 



A is the tube or retort containing the metal experi- 

 mented on in its lower extremity, and having a platinum 

 wire sealed into it at a distance of about two inches from 

 the lower end, the other end being drawn out and con- 

 nected by a mercury joint to an ordinary Geissler tube, 

 which is connected by another mercury joint to the 

 Sprengel pump c. 



Another form of tube which I have used is prepared 

 by inserting two platinum poles into a piece of combus- 

 tion tubing sealed at one end, and after inserting the 

 metal to be experimented on, drawing out the glass 

 between the platinums to a capillary tube. 



I have also tried inserting the platinum pole at the end 

 of the retort, so that the spark passes from the surface of 

 the metal, but this arrangement did not answer at all. 



Some other modifications have been tried, but the first 

 form I have described is that which I have found to 

 answer best, so far as the trials have yet gone. 



D is the spectroscope. 



E is the lens used for focussing the image of the Geissler 

 tube on the slit. 



F is the spirit lamp for heating the retort. 



H is the battery. 



K and L are the wires connected with the coil. 



In the second cut (Fig. 3) the method of observing the 

 spectrum of the vapours close to the surface of the metal 

 is indicated ; the same letters apply, D' being, however, in 

 this case a direct-vision spectroscope, which was some- 

 times employed for convenience. 



For determining the exact positions of the lines in the 

 spectrum of the vapour in any part of the retort, a larger 

 spectroscope, with its illuminated scale, was used in the 

 place of the direct-vision spectroscope. 



The secondary wires of the coil were connected, one 

 with the pole in the upper bulb at B, and the other with 

 the platinum at A. 



B is an ordinary Geissler tube with two bulbs separated 

 by a capillary tube. The great advantage of this arrange- 

 ment is that this capillary portion can be used for ascer- 

 taining what gases or vapours are carried over by the 

 pump without any interference with the retort, both wires 

 being connected with the Geissler tube. If, for example, 

 we are working with sodium which cofitains an impurity 

 of hydrocarbon, the moment at which it begins or ceases 

 to come off can be found by e.xamining the spectrum of 

 this capillary tube. 



I now give an account of the phenomena observed 

 when we were working with sodium, in order to show the 

 kind of phenomena and the changes observed. 



After a vacuum has been obtained the retort is heated 

 gradually. The pump almost immediately stops clicking, 



