December 2, 1895.] 



KNOWLEDGE. 



283 



which give the spectra. If the same spectrometer is used 

 throughout our observations, we can draw up maps of 

 spectra very readily by noting the position of the observing 

 telescope on the graduated scale of the instrument when a 

 definite line is on the cross wires of the eye-piece. 



In careful comparisons a still better and more certain 

 method than comparing maps is to observe the two actual 

 spectra at the same time, one being caused to occupy a 

 position above the other. If the same lines occur in both 

 the spectra they are seen at once to overlap, or some of 

 the lines in the upper spectrum are continued in the 

 lower. The upper spectrum may be that of a known 

 element, the presence of which in the second substance'is 

 being tested. In order to cause one spectrum to fall above 

 the other in this way, many spectroscopes are furnished 

 with a small right-angled prisai, which by means of internal 

 reflection at one of its faces causes the spectrum from a 

 neighbouring source of light to be thrown above that of 

 the flame placed in front of the slit . 



This method of analysis by means of observations on the 

 spectra of flames containing substances in the form of 

 incandescent gases gives a test of extraordinary delicacy 

 for the presence of elementary substances. The method 

 far surpasses those formerly used in chemical analysis. 

 The presence of sodium in the ordinary air we breathe is 

 shown quite clearly when we observe a colourless flame, 

 like that of the Bunsen lamp, through a spectroscope. 

 The yellow lines of the sodium spectrum shine out when 

 the flame is burning quietly by itself, although the quantity 

 of sodium present must be extremely small, coming as it 

 does merely from the salt borne by the wind from the 

 tossed up foam of the sea. In fact, the presence of the 

 three millionth part of a milligramme, or the one hundred 

 and eighty millionth of a grain of sodium, in a flame can 

 be detected by spectroscopic observations. The usual 

 occurrence of the yellow line in various spectra puzzled 

 chemists for long, and some supposed it must be due to a 

 substance universally present, such as water ; but Prof. 

 Swan showed its true source, and its practically constant 

 occurrence may be put down to the fact that no part of 

 our country is very distant from the sea. By means of 

 spectrum analysis, many substances such as lithium, which 

 were considered of comparatively rare occurrence, have 

 been shown to be present in many mineral waters. 

 Lithium itself has been found Ln vegetable structures and 

 in the human body, in the blood and musciUar tissue, and 

 may play a formerly unsuspected part in the economy of 

 nature. 



Amongst the rarer elements, whose existence was 

 unknown until they were detected by means of the 

 spectroscope, are cresium, rubidium, indium, thallium, 

 and gallium. Cipsium and rubidium were discovered by 

 Bunsen in 1860. Rubidium is named from two splendid 

 deep red lines which are given by its vapour. Ca?sium 

 derives its name from the two characteristic blue lines of 

 its spectrum. These two metals have since been found 

 in the water of many mineral springs, and rubidium is 

 widely dift'used, being found in beetroot, in tobacco, and 

 in coffee, tea and cocoa. 



In spectroscopic observations on gases such as hydrogen 

 and oxygen, electric discharges from the terminals of an 

 induction coil are sent through a tube of the rarefied gas, 

 which is thus caused to give out a luminous glow. In the 

 case of metals which are volatilized with difliculty, the 

 electric arc is made use of, the metals being exposed to the 

 intense heat between the carbon points. The spectra of 

 gases vary in many cases when the pressure to which the 

 gas is subjected is changed. 



It was observed by WoUaston, and afterwards by 



Fraunhofer, that when sunlight is drawn out into a 

 spectrum by means of a prism, the broad band of colour 

 obtained is not strictly continuous, but is traversed by 

 dark lines, indicating the absence of rays of certain 

 refrangibilities. This peculiarity was also found to exist 

 in the spectra of many of the fixed stars. The reason of 

 the appearance of these dark lines for long remained 

 mysterious, and the mystery was rather increased than 

 diminished when Fraunhofer observed that the bright 

 double line produced by incandescent sodium vapour coin- 

 cided in refrangibility and in position in the spectrum with 

 the double dark line called D in the solar spectrum. A 

 step towards the solution of the difficulty was made when 

 Sir D. Brewster showed that analogous lines were pro- 

 duced when a jar of nitrous acid gas was interposed in 

 the path of a ray of light. This experiment seemed to 

 indicate that the dark lines were not due to the absence of 

 certain rays from the light as it was originally given out 

 by the sun, but were caused by the absorption of certain 

 rays of definite refrangibility by vapours which the sunlight 

 passed through before reaching the observer. Whether 

 the vapours producing this absorption and cutting off of 

 part of the rays were situated in the atmosphere of the 

 sun or in that of the earth, remained unsettled until the 

 question was finally cleared up by the experiments of 

 Kirchhoft" in Germany. The insight of Sir G. G. Stokes 

 had previously suggested the true explanation, though this 

 suggestion seems to have been unknown to Kirchhoff, 

 whose experiments were carried out quite indepandently. 



Kirchhoff' found that when light from incandescent 

 sodium passed through sodium vapour at a lower tempera- 

 ture, the characteristic yellow rays were absorbed by the 

 vapour. Gases or vapours which are capable of giving out 

 certain rays when they are sufficiently raised in tempera- 

 ture, are also capable of absorbing these same rays. This 

 property is analogous to that which occurs in experiments 

 on sound. A tuning-fork or stretched wire which can 

 vibrate so as to give out a note of definite pitch when it is 

 struck, can also absorb or take up this vibration from the 

 surrounding air when a fork or wire giving the same note 

 as itself is sounded near it. When a source of light of 

 great intensity, such as the Hght from incandescent lime 

 or that of the electric arc, is placed behind a sodium flame 

 whose temperature is lower, it is found that the continuous 

 spectrum formed by the light which has passed through 

 the sodium vapour has a dark line in the place of the 

 yellow line D which the sodium vapour itself gives out. 

 The appearance is as if there was a narrow dark gap 

 at the position of the D line in the spectrum which is 

 otherwise continuous. In the case imagined, the dark 

 line does not denote entire absence of these rays from 

 the spectrum, but merely that the light at that part 

 is much less intense than that from the surrounding 

 portions, producing thus the effect of darkness in com- 

 parison with the rest of the spectral band. In fact, the 

 light from the dark line is of the same intensity as that 

 which would be given if the sodium flame was observed 

 alone, the intense light behind it being removed. The 

 sodium vapour absorbs those particular yellow rays which 

 it can itself give out, the light reaching it beinj, after 

 transmission, destitute of such ; but the vapour, being 

 itself incandescent, gives out on its own account rays 

 occupying the same position as those which it absorbad 

 and filter'ed out of the light reaching it. Thus the dark 

 line is lit up only by light from the sodium flame, which 

 being much less intense than that from the powerful 

 electric arc, seems black in comparison. If a sodium 

 flame sends light of the same intensity through that of 

 another sodium flame, that is, if we take two flames of the 



