796 



DIATHERMANCY 



DIATOMS 



hot water to two parts of ground malt (see BEER), 

 or freshly germinated barley, and, after standing 

 for a short time, straining through a cloth. The 

 proportion of diastase in malt is not more than 

 1 part in 500 parts, and yet it performs important 

 functions. Thus diastase has a powerful action 

 upon starch, and at a temperature of 150 F. one 

 part is considered powerful enough to change 2000 

 parts of starch into dextrine, and then into maltose, 

 a variety of sugar. When obtained separately, 

 diastase is a white tasteless substance, soluble in 

 water and in weak alcohol, and having no action 

 on gum or sugar. A similar principle is found in 

 the saliva of animals, the action of which on starch 

 is identical with that of diastase. When bread is 

 masticated, the saliva is rapidly secreted, and the 

 animal diastase converts the starch into sugar. See 

 GERMINATION, COOKERY, DIGESTION. 



Diathermancy. See HEAT. 



Dia' thesis (Gr. dia, 'through,' and tithcmi, 

 'I place or arrange'), a Greek word signifying a 

 disposition or arrangement, and applied by the old 

 medical authors to the predisposition or constitu- 

 tion of the body which renders it prone to certain 

 diseased states. By recent writers the term is 

 applied to the general constitutional tendency of 

 an individual, to indicate not merely the class of 

 diseases to which he is most likely to be liable, 

 but also in many cases the manner in which his 

 normal functions, both bodily and mental, are 

 carried on ; and is thus opposed to Cachexia (q.v. ), 

 an unhealthy condition associated with actual 

 disease. Though the study of diathesis apart 

 from existing disease is very apt to lead to over- 

 refinement and the pursuit of intangible abstrac- 

 tions, yet a proper appreciation of a person's con- 

 stitutional proclivities or diathesis often enables a 

 medical man to advise him how to regulate his 

 life and habits to the best advantage what to do 

 and what to avoid and may furnish important 

 guidance as to his treatment in disease. The 

 diatheses most usually recognised are the sanguine, 

 the nervous or neurotic, the bilious, the lymphatic, 

 the strumous, and the haemorrhagic. Numerous 

 mixed varieties are also described by some writers. 



Diatoms (Dtatotnaceee) are a group of alg;e 

 which, on account of their microscopic interest 

 and geological importance, have acquired an un- 

 usual share of scientific and even popular atten- 

 tion. They were discovered by Leeuwenhoek in 

 1702, and their movements by O. F. Miiller eighty 

 years later ; their thorough investigation, however, 

 has only become possible with the development of 

 the compound microscope. 



The reader who wishes to view the diatoms 

 other than as mere microscopic marvels must 

 begin with a clear grasp of the structure and 

 mode of life of the filamentous conjugate algee, 

 such as Spirogyra, and next observe that of those 

 higher members of the group which we know as 

 Desmids (see ALGJE, DESMIDS). These simpler 

 forms understood, let him next imagine a desmid 

 (in which the characteristic division of the uni- 

 cellular body into halves is distinctly but not too 

 deeply marked) to become somewhat unequally 

 developed ; next let these be pressed together so 

 that the larger half slides a little way over the 

 smaller, much like the lid of a canister or the 

 halves of a pill-box. Let the two halves or shells 

 of the cell-wall now become strongly silicified, 

 the cellulose only remaining unaltered and flex- 

 ible round the narrow ' girdle band ' connecting 

 them ; next let variation arise in the general shape 

 so that the original box-shape becomes elliptical 

 or wavy, squared, or more often pointed, or even 

 unsymmetrically curved ; finally, let the siliceous 

 shells become covered with the most delicate 



striations and markings, and these characteristi- 

 cally varied, not only from group to group, but 

 from species to species. A gelatinous envelope 

 may also be developed, or this may be secreted at 

 one pole only, forming a stalk. 



The living protoplasm shows less variation than 

 might perhaps have been expected ; it lines the 

 siliceous shell, leaving a large central sap-cavity, 

 often traversed by protoplasmic filaments. Very 

 commonly, however, this is divided by a large 

 central mass of protoplasm usually containing the 

 nucleus, while similar accumulations may be formed 



Fig. 1. Fig. 2. 



Pinnulnria viridis : 1. a, Optical section through the unequal 

 valves ; 6, side of one valve, showing markings and longi- 

 tudinal slit. 



2. Diagrammatic transverse section ; pp, protoplasm contain- 

 ing endochrome-plates, ep; g, girdle band. Valves left un- 

 shaded, the left showing depressions due to markings. 



at the ends. The colouring matter may occur in 

 minute granules, or be collected into one or two 

 large ' endochrome-plates ; ' it consists of chloro- 

 phyll, masked by a closely allied yellow pigment 

 (phycoxanthin). Starch is absent; oil is fre- 

 quently present, either in minute vacuoles or 

 collected into a single large drop. 



The mechanism of the peculiar creeping or 

 rather gliding movements has long baffled investi-' 

 gators ; these, however, are not due to diffusion 

 currents as some have maintained, nor to the 

 agency of any ordinary cilia or pseudopodia, but 

 seem to be effected by means of a locomotor band 

 of protoplasm which is said to be protruded 

 through a longitudinal slit in the surface of the 

 siliceous shells (see fig. 1). 



The mode of multiplication is primarily by 

 division, and is effected on the same principle as 

 in Desmids (q.v.). Thus the two halves of the 

 diatom are not only of unequal ages, but since the 

 new half is always formed within the previous 

 one, a continuous diminution of size takes place. 

 At a certain limit, however, division stops, and 

 rejuvenescence may occur, with formation of a 

 resting spore ; more frequently, however, this is 

 preceded by conjugation as in desmids, though 

 sometimes complete union may not take place. 

 The resultant ' auxospore ' has a continuous cellu- 

 lose coat, but develops within it, by rejuvenes- 

 cence, a two-shelled diatom of the largest size, 

 which issues to divide in turn. 



Of the 2000 species, 400 are fresh-water, the 

 remainder marine. Their distribution is ubiqui- 

 tous, and the genera and even species seem little 

 dependent upon temperature or climate, many 

 being apparently cosmopolite, and some having 

 been described as occurring in glacier water, yet 

 also in hot springs. Their minute size and resist- 

 ance to drying favour their distribution in the 

 form of dust ; hence the calcination of the dust 



