G32- 



THE GAltDENEIiS , CHlWNiCLE. 



[December 1, 



Analysis with the combustion-tube, as mentioned 

 above, which gives the carbon of a compound as car- 

 bonic acid, and hydrogen as water, makes known the 

 total amount of carbon and hydrogen in the weighed 

 quantity examined, but it does not tell anything 

 about the molecular arrangement. This is work yet 

 to be done. 



The polariscope, to which brief reference must be 

 made, has revealed to us that bodies having the same 

 chemical composition (as regards weight proportion) 

 may have very different physical properties. The 

 natural inference is, that they are not identical in 

 the form of their composition. 



Perhaps the polariscope is not familiar to all 

 cultivators, even under its trade name, saccharometer, 

 which is one form of it. Most people, I suppose, 

 have seen Iceland spar in an optician's window. There 

 is generally a wafer or spot of colour under it. In 

 certain directions this appears double. Now, without 

 entering into questions of optical axes, or the wave 

 theory of light in connection with the subject, what 

 has been found can be done is this. Leaving out all 

 details (which, with the theoretical explananation 

 can be found in popular form in Mr. Spottiswoode's 

 lectures to his workpeople), *a slice of the spar cut in a 

 particular direction is fixed in a tube which admits 

 light on to it. There must be no other light in the 

 room powerful enough to interfere with what has to 

 be watched. Another slice is in another tube, so 

 fixed that it can be turned round. The two tubes are 

 placed so that the light, after passing the first, passes 

 on through the second to some white surface. Some 

 coloured light is used — say yellow. The light is seen 

 on the white surface — a white wall, or sheet of paper. 

 Then the further tube is gradually turned round, 

 still kept in the position that the light enters it ; but 

 as it is turned, and with it of course the slice of 

 spar, the light fades till it disappears. This takes 

 place at half a full turn round. 



If a glass vessel with flat sides containing dis- 

 solved sugar be then placed between the two tubes 

 the light will reappear, passing on to the white 

 surface. But a certain amount of turning of the 

 tube will stop it again. 



Different solutions are found to have different 

 effects in this respect, and they have been care- 

 fully examined with apparatus, in which the 

 amount of turning given is indicated on a 

 scale marked off in degrees. They have also 

 been examined with lights of different colours, 

 and in this way their action on what is called 

 polarised light is known. Now, with regard to 

 the action of solutions of the carbo-hydrates it has 

 been found that even those which have the same 

 chemical constitution behave differently. Just as at 

 a meeting it all depends on whether you are on the 

 platform or in the hall which you call right or left, 

 so it depends on whether you are in front of or 

 behind the tube you would say it turned right or left. 

 But it is conventionally agreed which is called right 

 and left. The practical point is that some solutions 

 give the light a " right hand twist "and some a "left 

 hand twist." The second tube has to be gradually 

 turned right or left to gradually cut off the light or 

 lit it pass again. I hope this makes the facts clear, 

 though the language is far from scientific. But then, 

 as regards using scientific language, we know what 

 ■we mean by a Potato-apple, though it is not an 

 Apple, and even " tuber " is not a proper word to 

 use if it is rightly used for some plants other than 

 Potatos. 



However, leaving out questions of " specific rota- 

 tory power," and the numbers used to express this, 

 the broad fact is, that some turn the light to the 

 right and some to the left. The plus sign ( + ) is used 

 for right and the minus sign ( — ) for left. Dextro 

 and lsevo are sometimes used in naming combinations. 

 Now let us look at what has been found with regard 

 to some of the carbo-hydrates. 



Among the glucoses, all of which have the same 

 chemical composition, as given above, dextrose 



(Grape sugar) is +, while levulose (fruit sugar) is — ; 



* W. Spottiswoode, F.E.S., Polarisation of Light : Nature 

 series. 



some others are + or — . Among the amyloids, 

 starch is + , while inuline is — , and dextrine is very 

 powerfully + . Temperature affects " rotatory power." 

 Composition, as shown by weight proportions, is not, 

 then, a full explanation of differences. Though the 

 polariscope is used in testing the strength of sac- 

 charine solutions, I am not aware that it has been 

 applied in starch manufacture. 



What is the significance of such facts as these 

 appears to have received but little attention. What 

 their study may in time practically lead to it is 

 impossible to guess. What is the physiological 

 effect between + and a — sugar is not known. It 

 has been wantonly suggested that babies cry when 

 their milk has — sugar, and laugh when it has + . 

 It has also been suggested there is a + Potato 

 starch and a — , and that both are grown in Ireland, 

 and give a right or left-handed light in which public 

 events are viewed. This can hardly be considered a 

 serious political suggestion, for there is no hint 

 whether it is the Unionist or Home Rule districts 

 that furnish the hypothetical — starch. 



Before passing on from this part of our subject, 

 there are one or two points to mention. 



Though both starch and dextrine belong to the 

 group amyloids, that is, they have the H and the O 

 present in the same proportion, and both are + 

 starch forms with iodine a deep blue compound, 

 which dextrine does not. Dextrine is called British 

 gum, and can be formed from starch when heated to 

 about 150° C. 



Cellulose, the colourless material of the woody 

 fibre of young plants, has the same composition, but 



l I I I I I I I I I I I I I I I I 



FIG. 88.— CHLOROPHYLL SPECTRUM. 



it behaves differently with sulphuric acid, or on 

 boiling. 



The addition of nitric acid and water to starch 

 affords an example of a substitution product. The 

 acid is NO„ added to starch C la H„ O 10 ; the result 

 is C lr! , H 10 "(NO 2 ), 10 , where N(\ has substituted 

 the place of our H. 



Reference has been already made to some early 

 observations on the effect of light on plants in decom- 

 posing the carbonic acid of the air, and so obtaining 

 their carbon. The connection of this with the 

 influence of chlorophyll has in recent years received 

 considerable attention, especially with the aid of the 

 spectroscope. Though the latest forms of this 

 instrument are complicated and costly, the principle 

 may be readily understood. It is easy to do as 

 Newton did, let light into a dark room through a 

 circular hole in a shutter, and hold a glass prism in 

 the path of the light. An ordinary candelabra drop 

 does very well. The coloured spectrum formed on a 

 wall beyond is no doubt familiar to everybody. 

 Wollaston used a straight slit instead of a round 

 hole, and got his spectrum more clearly defined. 

 What is seen is what is called in ordinary language 

 "all the colours of the rainbow." In a spectrum 

 caught on white paper from a good prism in strong 

 light it is not difficult, with attention, to see there 

 are dark shades or lines in various parts of the 

 spectrum. It is on the presence of these lines that 

 the work of spectroscopy is based. Quartz prisms 

 or a wedge-shaped bottle containing a solution of 

 bisulpdide of carbon, is used instead of a glass 

 prism ; the source of light is in a lantern, and 

 a tube with lenses collects and focuses the 

 light on to a narrow slit. The electric light 

 gives the spectrum complete, but different bodies are 

 found on ignition to give only parts, in some cases 

 very small parts of it, bright bands only here and 

 there. The position and number of these bands 



differs for different bodies, but is always constant for 

 the same body. Hence it has been possible, taking the 

 complete spectrum as a standard and marking it off 

 into degrees (see fig. 88), to map the position of the 

 bands and lines which occur in spectra from different 

 sources. So characteristic are these that by looking at 

 the spectrum any one familiar with this kind of work 

 can tell what body or mixture of bodies is being used 

 as a source of light. Different solutions also put in the 

 path of the light are found to " cut out " — obliterate — 

 part of the spectrum ; and as the behaviour of so 

 many has been studied it is possible to recognise 

 certain solutions in this way. The applications of 

 spectrum analysis at the present day are many — from 

 reading the constitution of heavenly bodies to 

 detecting the presence of minute quantities of bodies 

 which ordinary analysis would fail to do. In plant 

 physiology its two chief uses are a study of the 

 effects of different portions of the solar spectrum, 

 and of solutions obtained from different parts of 

 plants. 



It was to the development of the daguerrotype 

 process of taking pictures we are largely indebted 

 for the impetus given to the study of the action of 

 different parts of the solar spectrum. 



Recent Work. 



There have been already mentioned : — 



a. The discovery of " fixed air." 



b. The recognition by Lavoisier that this is 



and its being named carbonic acid. 



c. The recognition of the distinction between 



0=0=0 CO, and >0=0 CO. 



d. The many investigations of the composition of 



the atmosphere. 



e. The determination of the fact that plants obtain 



their carbon from the C0 3 of the atmosphere. 

 /. That intensity of light has an influence on the 



amount of carbon fixed. And 

 ff. That the colour of light has also an influence. 



It now remains to notice the more recent work 

 which has been effected with the refined appliances 

 which modern science has devised. This work has 

 in part confirmed and in part modified the conclusions 

 drawn from earlier work. It has narrowed the 

 questions still under discussion and given to them 

 greater precision. Much of our increased knowledge 

 of the various forms of solar energy has come 

 from the subjects being taken up from a purely 

 physical point of study. Starch and its modifications 

 has also been studied from a purely chemical point of 

 view. But it is with it only in connection with the 

 growing plant we are at present concerned. 



Recent work has mainly centered round the 

 questions : — 



1. What is the true function of chlorophyll ? (chloro- 



phyll is mentioned above under Senebier) and 

 what is it ? 



2. What is the connection between chlorophyll, 



light action, and starch formation ? 



3. Is starch the first carbon compound formed in a 



plant ? 



4. What is the constitution of starch-forming bodies 



themselves ? 



Around these are clustered many subsidiary ques- 

 tions. 



Memoirs and papers on these points are numerous, 

 all of them, however, from foreign sources. Many 

 of their questions overlap, and are treated in the 

 same paper, and the work on the different lines has 

 been going on more or less simultaneously. A 

 retrospect in chronological order is therefore hardly 

 possible. 



As already mentioned, Senebier drew attention to 

 connection of plant growth and coloured light. 

 II. von Mohl first pointed out (1836) the common 

 association of starch with chlorophyll, and though 

 but vaguely, this was enough to direct attention to 

 the subject. Subsequently Bohm, with improved 

 iodine test, found a means of more easily determining 

 its presence in chlorophyll corpuscles. From gradu- 

 ally extending examination of a wide range of 



