434 THE CHEMICAL COMPOSITION OF THE BODY [CH. XXVIII. 



4. Gliadins ; the proteins soluble in alcohol just alluded to. They are character- 

 ised also by the absence of lysine among their cleavage products, and usually yield 

 a very high percentage of glutamic acid on decomposition. The gluten of wheat 

 flour, which is formed when water is added to it, has been shown to consist of two 

 proteins one (gliadin) soluble in alcohol, the other (glutenin) soluble in alkali. It 

 is to the former that the gluten of dough owns its cohesiveness ; and grains such as 

 rice, which contain no gliadin, cannot in consequence be employed for making 

 bread. 



The Polarimeter. 



This instrument is one by means of which the action of various substances on 

 the plane of polarised light can be observed and measured. Most of the carbo- 

 hydrates are dextro-rotatory. All the proteins are laevo-rotatory (see p. 425). 



There are many varieties of the instrument ; these can only be properly studied 

 in the laboratory, and all one can do here is to state briefly the principles on 

 which they are constructed. 



Suppose one is shooting arrows at a fence made up of narrow vertical palings ; 

 suppose also that the arrows are flat like the laths of a Venetian blind. If the 

 arrows are shot vertically they will pass easily through the gaps between the 

 palings, but if they are shot horizontally they will be unable to pass through at 

 all. This rough illustration will help us in understanding what is meant by polarised 

 light. Ordinary light is produced by the undulations of the aether occurring in all 

 directions at right angles to the path of propagation of the wave. Polarised light 

 is produced by undulations in one plane only; we may compare it to our flat 

 arrows. 



In a polarimeter, there is at one end of the instrument a Nicol's prism, which 

 is made of Iceland spar. This polarises the light which passes through it ; it is 

 called the polariser. At the other end of the instrument is another called the 

 analyser. Between the two is a tube which can be filled with fluid. If the analyser 

 is parallel to the polariser the light will pass through to the eye of the observer. 

 But if the analyser is at right angles to the polariser it is like the flat arrows hitting 

 horizontally the vertical palings of the fence, and there is darkness. At inter- 

 mediate angles there will be intermediate degrees of illumination. 



If the analyser and polariser are parallel and the intermediate tube filled with 

 water, the light will pass as usual, because water has no action on the plane of 

 polarised light. But if the water contains sugar or some ** optically active " substance 

 in solution, the plane is twisted in one direction or the other according as the sub- 

 stance is dextro- or laevo-rotatory. The amount of rotation is measured by the 

 number of angles through which the analyser has to be turned in order to obtain 

 the full illumination. This will vary with the length of the tube and the strength 

 of the solution. 



The Lipoids. 



This name was first applied by Overton to a heterogeneous group 

 of substances found in the protoplasm of all cells, especially in their 

 outer layer or cell-membrane, which, like the fats, are soluble in such 

 reagents as ether and alcohol. These substances, though present in 

 smaller amount than proteins, appear to be essential constituents of 

 protoplasm, and the labile character of their molecules is a property 

 many of them share in common with the proteins. 



The lipoids are found mixed with fat in the ether-alcohol 

 extract of tissues and organs, and they are specially abundant in 

 nervous tissues. They can be separated by what is called selective 

 extraction. For instance, cold acetone will dissolve out only choles- 

 terin ; hot acetone then dissolves out a mixture of substances named 



