December 7, 1917] 



SCIENCE 



551 



(as well as an apical leaf) send out sub- 

 stances toward the base of a stem which 

 inhibit the buds from growing out. These 

 inhibitory substances may be identical with 

 or may aceompanj'- the root-forming hor- 

 mones. The most apical bud in an excised 

 piece of stem will grow out first since it 

 will be the first to be free from these inhib- 

 itory substances. 



In a former paper the writer had pointed 

 out that a leaf sends out substances, in an 

 apical direction through the stem, which 

 favor shoot formation. 



Jacques Loeb 



The Eockeeeller Institute fob 

 Medical Research, 

 New York 



SOME COMMENTS ON THE THEORIES 

 OF THE STRUCTURE OF MATTER i 



Professor Lewis in his paper raised the 

 question of valence. From the point of view 

 of chemistry, valence has a defiuite meaning 

 which can not be overlooked and which 

 may be emphasized here. The conception 

 of valence developed from a study of the 

 regularities observed in the composition of 

 substances, and is fundamentally purely de- 

 scriptive. It is a classification which shows 

 regularities in the capacity of certain atoms 

 for combination, or for holding a definite 

 number of atoms or their equivalents in 

 combination. The continued study of 

 chemical composition has, as a matter of 

 course, extended the classification. The 

 phenomena of oxidation, the ionization of 

 substances in solution and otherwise, and 

 similar properties, have brought forward 

 the view that, choosing a suitable element 

 or state of an element as the zero or neutral 

 point, the valence of an element in a given 

 combination may be denoted either by a 



1 This discussion was presented by Dr. Palk at 

 the "Symposium on the Structure of Matter," 

 held at the meeting of the American Association 

 for the Advancement of Science in New York City, 

 December, 1916. 



positive number or a negative number. 

 This view was adopted for individual cases 

 some time ago by different chemists, but 

 became of general interest when J. J. Thom- 

 son, using corpuscles, showed how this could 

 be pictured readily, and applied in a simple 

 manner. 



A few words may be devoted to the fact 

 that the classifications given by valence 

 should involve no considerations of meas- 

 ures of relative stabilities of substances, al- 

 though the existence of compounds depends 

 upon stabilities and rates of decomposition. 

 Stability discussions should not enter di- 

 rectly into questions of valence, but unfor- 

 tunately this fact is often overlooked and 

 much confusion has resulted. 



The question of so-called polar and non- 

 polar valence is one raised by Professor 

 Lewis. At the present time the view that 

 only non-polar bonds exist is probably held 

 by no chemist. The electron conception of 

 valence, based upon a study and compari- 

 son of organic and inorganic compounds, 

 postulates polar valence only; in other 

 words, each valence linking is equivalent 

 to one atom functioning with a negative 

 charge, and the other atom with a positive 

 charge. The electrostatic view does not in- 

 volve at first sight such questions as distri 

 bution of electrons within the atom, etc. 



At the present time there are a numbe> 

 of chemists who advocate both polar and 

 non-polar valences, even assuming both to 

 be present in a molecule at the same time. 

 The reasons for assuming the existence of 

 non-polar valences appear to be negative 

 ones. If direct evidence is lacking, or if 

 ignorance is manifested with regard to the 

 reactions of certain groups, or if these 

 groups do not take part in the desired re- 

 action with sufScient velocity, the existence 

 of polar valences is denied. A strong argu- 

 ment in favor of assuming polar valences 

 in organic compounds is, that if they are 



