September 2, 1922] 



NA TURE 



309 



Letters to the Editor. 



[The Editor does not hold himself responsible for 

 opinions expressed by his correspondents. Neither 

 can he undertake to return, or to correspond with 

 the ■writers of, rejected manuscripts intended for 

 this or any other part of NATURE. No notice is 

 taken of anonymous communications.] 



Spectrum Lines of Neutral Helium. 



Dr. Silberstein's letter in Nature of August 19, 

 p. 247, induces me to write to say that some time 

 ago I found the key for unravelling the constitution of 

 the secondary spectrum of hydrogen to be of a kind 

 similar to, though more generalised than, that used 

 by him for helium. Practically the whole of this 

 spectrum depends on the sequence of the Balmer series. 

 If f(m) denote the wth sequent, the wave number 

 of any line is of the form 1k,„f(m), where the k m 

 are positive or negative integers ; e.g. the line n = 

 1689272 is f(z) -/( 3 ) +/( 4 ) -/( 5 ) -/(6) +/( 7 ) = H„ + 

 (H-y-H )-(Hs-H f ) within an observation error d\ = 

 001. In fact the spectrum is a kind of linkage spectrum 

 in which the usual links are replaced by the separations 

 between the successive lines of the primary, namely, 

 533 x '57' 2 4 (, 7'75. etc - The same machinery of 

 analysis used for linkage spectra is then directly 

 applicable, but as the total number of observed lines 

 is about 1600 it may be understood that a considerable 

 time is required for the completion, arrangement, and 

 discussion of the various physical effects in different 

 groups of lines. The preliminary work of forming the 

 linkage maps is practically completed. The results 

 so clearly suggested that Curtis's helium spectrum was 

 built in the same way that I was on the point of 

 writing to him to suggest his testing them, and now 

 Dr. Silberstein's very interesting letter comes to show 

 independently that this is the case. 



It has always seemed to me that the existence of 

 these linkage spectra forms a difficulty in the orbital 

 theory of spectral lines. This difficulty Dr. Silber- 

 stein's theory does not meet. According to his theory, 

 and apparentlv in any orbit theory, the two electrons 

 are moving independently, and each passes between 

 two of its corresponding paths. But if the combined 

 change of energy is radiated, these two events must be 

 absolutely simultaneous, and would happen, say, once 

 in an ceon. W. M. Hicks. 



August 19. 



Micro Methods in the Practical Teaching of 

 Chemistry. 



A change in teaching methods which brings with 

 it simplicity and economy should appeal to all. May 

 I therefore direct attention to methods I have myself 

 begun in Cairo of teaching chemistry from the begin- 

 ning by " microchemical " methods — that is, by 

 working with very small quantities ? (We really 

 need a better word than " microchemical," which 

 seems to suggest the microscope.) 



It is strange that even in science we are so con- 

 ventional. Pieces of apparatus once introduced by 

 some one of repute remain in the shape and form, size 

 and weight in which they were first employed as if 

 they were consecrated obj ects. The Bunsen burner, for 

 example, although it may be far bigger than required 

 and very expensive in its consumption of gas, is 

 scarcely ever changed. Even where a far less con- 

 sumption of gas would suffice it seems to be the 

 tradition to burn a large quantity. It is the same 

 with the size of the test tube, beaker, or flask in 



common use, and the same also with the quantity of 

 material used by the student in carrying out his 

 chemical tests : he will as a rule take a quarter, if not 

 a half, test tubeful of some solution and add as much 

 of the testing reagents as he can get into the test tube. 

 There seems to be no necessity for these large quan- 

 tities, and most of the tests carried out by students in 

 chemical laboratories could be done with far less 

 material. I propose to describe some of the methods 

 which have been used in the Government Medical 

 School Chemical Department, Cairo, during the past 

 year, feeling that many laboratories would gain much 

 in time, materials, and money by following methods 

 which have proved very successful there. 



These methods arose out of necessity. Too many 

 students and too little space is probably a state of 

 things not special to Cairo ; the extreme was reached 

 when it became necessary to teach two hundred 

 students without any proper laboratory at all. 

 Rather than refuse admission to the students a solu- 

 tion of the difficulty was sought in microchemical 

 methods, and the result was successful beyond antici- 

 pation. Many who visited the laboratory at the time 

 were surprised to see a hundred students seated in 

 perfect silence busily engaged in applying micro- 

 chemical methods in a hall in which there was neither 

 water nor gas nor any of the appurtenances of a 

 chemical laboratory. The necessity for such an 

 improvisation could scarcely arise under more settled 

 conditions, nor for that reason is it likely that any one 

 would have the opportunity of carrying out such a 

 teaching experiment on so large a scale. 



The root idea of the method is economy in its 

 broadest sense : in time, labour, and materials. 

 Clearly if the student uses nothing larger than a drop 

 instead of the habitual inch or half -inch in a test 

 tube the expense in chemicals can be readily reduced 

 one hundredfold. The expense in students' chemicals 

 represented during the year only a few pounds of 

 materials, the consumption of most of which is to be 

 attributed to second-year students doing special work. 

 About 500^. has been saved out of chemicals alone, 

 which saving can be applied to the purchase of 

 permanent app'aratus. 



The economy in apparatus has been even greater 

 than in chemicals. The bottles throughout the 

 laboratory have been reduced to one-tenth of the con- 

 ventional' size. Each student is given at first a small 

 rack, and later another, containing six reagent bottles 

 of one ounce capacity. These bottles are unstoppered 

 but fitted with small dropping-pipettes. The bottles 

 are cheap and the pipettes are made in the laboratory. 

 The racks are easily collected and stored, and it is 

 possible in this way to keep a class constantly supplied 

 with freshly prepared solutions by issuing only those 

 reagents which it is intended to employ at the time, 

 thus avoiding the making up at the beginning of 

 term of large quantities of solutions which may not 

 be required till many months later. No test tubes 

 are used till the student comes to actual separation 

 of the groups, so that for the first half of the course all 

 the expense and annoyance of breakage, difficulties 

 of cleaning, and mess due to test tube work on the 

 benches is avoided. 



Indiscriminate test-tubing by students untrained 

 in delicate manipulation and without any quantita- 

 tive sense is, as it seems to me, a bad influence in 

 their training which it is important to combat. In 

 so many schools has it been the custom to use materials 

 in wastefully large quantities that the name " stinks " 

 only too aptly describes what goes on. But if the 

 student be taught from the outset to regard the drop 

 as a suitable, if not already a large quantity, he will 

 get nearer to the quantitative notion and may acquire 

 some of that delicacy of manipulation so essential to 



NO. 



757, VOL. IIO] 



