338 



NATURE 



[May 12, 1921 



to us as an "electron," its mate, which appears 

 to be the real unit of mass, has only recently 

 been given the name of "proton." 



The Rutherford atom, whether we take Bohr's 

 or Langmuir's development of it, consists essen- 

 tially of a positively charged central nucleus around 

 which are set planetary electrons at distances which 

 are great compared with the dimensions of the 

 nucleus itself. As has been stated, the chemical 

 properties of an element depend solely on its 

 atomic number, which is the charge on its nucleus 

 expressed in terms of the unit charge e. A 

 neutral atom of an element of atomic number N 

 has a nucleus consisting of K + N protons and K 

 electrons, and around this nucleus are set N elec- 

 trons. The weight of an electron on the scale we 

 are using is 00005, ^^ ^^^^ it may be neglected. 

 The weight of this atom will, therefore, be K + N, 

 so that if no restrictions are placed on the value 

 of K any number of isotopes is possible. 



The first restriction is that, excepting in the 

 case of hydrogen, K can never be less than N, 

 for the atomic weight of an element is always 

 found to be equal to, or greater than, twice its 

 atomic number. The upper values of K also seem 

 to be limited, for, so far, no two isotopes of the 

 same element have been found differing by more 

 than 10 per cent, of its mean atomic weight; the 

 greatest numerical difference is eight units in the 

 case of krypton. The actual occurrence of isotopes 

 does not seem to follow any law at present 

 obvious, though their number is probably limited 

 by some condition of stability. 



Protons and electrons may therefore be regarded 

 as the bricks out of which atoms have been con- 

 structed. An atom of atomic weight m is turned 

 into one of atomic weight m + i bv the addition 

 of a proton plus an electron. If both enter the 

 nucleus, the new element will be an isotope of the 

 old one, for the nuclear charge has not been 



altered. On the other hand, if the proton alone 

 enters the nucleus, and the electron remains out- 

 side, an element of next higher atomic number 

 , will be formed. If both these new configurations 

 are possible, they will represent elements of the 

 same atomic weight, but with different chemical 

 properties. Such elements are called "isobares," 

 and are actually known among the radio-active 

 elements. 



The case of the element hydrogen is unique, for 

 its atom appears to consist of a single proton as 

 nucleus with one planetary electron. It is the only 

 atom in which the nucleus is not composed of a 

 number of protons and electrons packed exceed- 

 ingly close together. Theory indicates that when 

 such close packing takes place" the effective mass 

 will be reduced, so that when four protons are 

 packed together with two electrons to form the 

 helium nucleus this will have a weight rather less 

 than four times that of the hydrogen nucleus, 

 which is actually the case. 



It is not to be supposed that the whole-number 

 rule is of exact mathematical accuracy, for the 

 unit of the oxygen scale is a "packed" pro- 

 ton + an electron, and its value will certainly alter 

 slightly with the degree of packing. On this 

 account it is of the greatest importance to push 

 the accuracy of methods of atomic weighing as 

 far as possible, for variations from the whole- 

 number rule, if they could be determined with 

 precision, would give us some hope of laying bare 

 that innermost of secrets, the actual configuration 

 of the charges in the nucleus. 



The results I have described he on the border- 

 line of physics and chemistry, and although as a 

 chemist I view with some dismay the possibility 

 of eighteen different mercuric chlorides, as a 

 physicist it is a great relief to find that Nature 

 employs, at least approximately, standard bricks 

 in her operations of element-building. 



Natural Camouflage. ^ 



THE fine volume under notice is a new edition 

 of the beautifully illustrated work which, 

 originally appearing in 1909, first brought in a 

 connected form before the public the many class- 

 ical principles of concealing-coloration estab- 

 lished by the genius of the American artist- 

 naturalist Abbott H. Thayer. Important dis- 

 coveries such as these, especially when the en- 

 thusiasm of their originator could recognise well- 

 nigh no limits to their application, were bound 

 to bring sharp differences of opinion. In America 

 we have seen the rise of two rival camps, one, 

 headed by the late Theodore Roosevelt, opposing 

 the whole of Thayer's conclusions, the other 

 accepting the whole and even interpreting the 



1 " Concealing-Coloratlon in the Animal Kingdom. An Exposition of 

 the Laws of Disguise through Colour and Patrern : Being a Summary of 

 Abbott H. Thayer's Disclosures." By Gerald H. Thayer. With an Intro- 

 ductory 'Essay by A. H. Thayer. New Edition with a New Preface. 

 Illustrated by Abbott H. Thayer and Others, and with Photographs. 

 Pp. xix-l-26o+xvi plates. (New York : The Macmillan Co- ; London : 

 Miu:millan and Co., Ltd., 1918.) ty. net. 



NO. 2689, VOL. 107] 



advertisement of Warning Colours and their 

 simulation in Mimicry as examples of the work- 

 ing, in one form or another, of concealing- 

 coloration. 



In England, where, as the result of the writ- 

 ings of Wallace and Bates, and still earlier of 

 Erasmus Darwin, tlie subject as a whole is older, 

 an intermediate position has been taken. Here, 

 naturalists recognise to the full the enduring value 

 and fundamental importance of Thayer's dis- 

 coveries, although believing that they do not offer 

 a complete interpretation of animal colouring as a 

 whole ; and. in the beautiful frontispiece of the 

 book, representing a peacock in the woods 

 with its blue neck against the sky and posed so 

 as to illustrate the conclusion that its pattern is 

 "a marvellous combination of ' obliterative * 

 designs, in forest-colors and patterns," in this and 

 the flamingoes, and spoonbills with "the skies 

 they picture " (plates viii-x), English naturalists 



