August 12, 1909] 



NA TURE 



207 



•dominating tliat which has to do with dead matter, while 

 carbon is the great organ-building- and maintaining 

 element of all living tilings. While each carries on the 

 work to which it is best suited under existing terrestrial 

 conditions, they both go about it in somewhat similar 

 ways, and each one shows a tendency to overstep the border 

 line and perform the other's part. This tendency is for 

 various reasons much more marked in the case of carbon, 

 but 1 hope to show you presently that silicon is by no 

 means out of touch with living things, and, further, that 

 it exhibits capacities which render it a potential element 

 ■of life under other conditions of our planet, but more 

 -especially at a much higher level of temperature. 



I do not propose to dwell in much detail on the remark- 

 able parallelism of some silicon and carbon compounds, 

 but must refer shortly to a few of them, and the oxides 

 naturally come first. 



The lecturer then described silicon oxide, chloride, 

 bromide, chloroform, &-c., with the analogous carbon com- 

 pounds, and continued : — Both silicon and carbon form 

 gaseous compounds with hydrogen of similar composi- 

 tion ; — 



CH, and SiH,. 



Neither of these hydrides can be obtained by direct 

 union of the respective elements, though they are easily 

 ■obtained by indirect means, with the details of which I 

 need not trouble you. Both are colourless gases, as you 

 see. The carbon hydride, or marsh gas, is combustible, 

 but requires to have its temperature raised considerably 

 before it takes fire in air, and its flame is only slightly 

 luminous. It produces on complete oxidation water 

 vapour and carbon dioxide' gas. The analogous silicon 

 hydride takes fire much more easily in air, and when not 

 quite pure is even spontaneously combustible under ordinary 

 conditions, and it burns, producing water vapour and solid 

 silicon dioxide. 



" Silico-organic Chemistry." 



Now, Just as marsh gas may be regarded as the start- 

 ing point of that great branch of science which is usually 

 spoken of as organic chemistry, so the analogous hydride 

 of silicon is the primary compound from which many 

 substances, which are often termed silico-organic com- 

 pounds, can be derived by various means, and these were 

 discovered in the course of the classical researches of 

 Friedel, Crafts, Ladenburg, and others. 



I wish to avoid using many chemical formuL-E, which 

 probably would convey but little meaning to some of those 

 whom I address ; it will suffice merely to indicate the 

 lines on which investigations have proceeded in this direc- 

 tion. 



In the older work of Friedel, Crafts, and Ladenburg, 

 they produced complex substances by the substitution of 

 various radicles falways carbon groups), for one atom of 

 hydrogen in SiH,. and ultimately replaced another atom 

 of hydrogen by the OH or hydroxyl group. The sub- 

 stances so formed were silicon alcohols, which may be 

 represented in the following manner. A, B, and C being 

 used to indicate the different complex replacing radicles : — 



Si 



-TI 

 -H 

 H 



A 



S'-H 

 OH 



S-^-H 

 S'-H 



OH 



SiZ 



H 



OH 



In this way silicon alcohols were built up which proved 

 to be analogous to well-known carbon alcohols, and 

 alTorded analogous acids, &c., on o.xidation. These dis- 

 •coveries laid the foundations of a silico-organic chemistry, 

 and have been further extended in later years. For 

 ■example, it has been found possible to pursue the analogy 

 with known carbon compounds in the direction of re- 

 placing all the hydrogen in silicon hydride by different 

 radicles, and these changes, which can be effected in 

 successive stages, may be represented in harmony with 

 those just given : — 



NO. 2076, VOL. 81] 



The two last of these are asymmetric, since all four 

 radicles are different. Consequently, they should exist in 

 two isomeric modifications if really analogous to known 

 carbon compounds of the same order, and each form should 

 be capable of acting differently on polarised light.' Dr. 

 F. Stanley Kipping, who has specially investigated this 

 kind of substitution with much success, finds that the 

 analogy between these asymmetric silicon and carbon com- 

 pounds is complete in regard to optical activity as to other 

 general characters. 



Silicon Compounds including Nitrogen. 



This was all good so far as it went, but some highly 

 important information was still wanting. As you know 

 well, the various compounds including carbon and 

 nitrogen play by far the most important parts in building 

 up organised structures under the influence of vital energy, 

 but in the silicon series we were almost wholly ignorant 

 of the existence of such compounds until within recent 

 years, when I undertook definitely to investigate this 

 branch of the subject. 



All that was known at the period of which I speak was 

 that silicon forms a white nitride of uncertain composi- 

 tion when strongly heated in an atmosphere of nitrogen 

 gas, and that when silicon chloride is brought in contact 

 with ammonia and similar substances violent action occurs, 

 but the nature of the products formed was not known 

 owing to special practical difificulties in separating them. 



The first step taken was to examine the action of silicon 

 halides (i.e. chloride, bromide, &c.) on substances free 

 from oxygen, but rich in nitrogen. The earliest of these 

 worked with were thiocarbamides, but in all these cases 

 the silicon halide merely united with the nitrogen com- 

 pound as a whole, in some instances producing very curious 

 substances, of which the one with allyl-thiocarbaniide, 



(C3H,.H,NXS),SiBr„ 

 is a good example. This is a liquid which flows so 

 slowly at ordinary temperature that it requires nearly a 

 month in order to fall from the top of its containing tube 

 and find its level at the bottom. Several similar sub- 

 stances have been obtained and examined, and their pro- 

 ducts of decomposition studied, but they do not belong to 

 the class of which I was really in search. 



It would weary you to give the details of scientific 

 prospecting which one has to go through in order to attain 

 definite results in a new line of work like this ; suffice it 

 to say that success attended the efforts at last, and a 

 finely crystallised and perfectly defined compound was 

 obtained in which silicon is wholly in direct chemical com- 

 bination with nitrogen, and a specimen of that substance 

 I now show you. Its composition is represented by the 

 expression 



Si(NHPh)„ 



where Ph stands for the phenyl group, and its name is 

 silicophenylamide. 



This substance when heated undergoes some important 

 changes, which resemble rather closely similar changes 

 that can be effected in analogous compounds of carbon 

 with nitrogen. Thus it first affords a guanidine, 



NHPh 



Si=NPh , 



■NHPh 



analogous to the well-known carbon guanidine, and further 

 a di-imide, Si(NPh),, which only needs the addition of a 

 molecule of water to convert it into a silicon urea, 

 SiO(NHPh),. Many other substances have been produced 

 similar to "silicophenylamide, and they afford analogous 

 products to these just mentioned ; but these have been 

 fully described elsewhere, and need not be dealt with 

 here. 



Silicon in Relation to Organised Structures. 

 The general results of these researches are that we now 

 know a considerable number of silicon compounds in- 

 cluding nitrogen, which resemble those of carbon with 

 nitrogen, both in composition and in the general nature 



1 These changes are rpprescnted above as having been effected ihrongh 

 Ihe silicon alcohols in order to avoid complicating the gener.al statement ; 

 other compounds have, in fact, been fourd more convenient for the purpose. 



