302 WHAT IS LIFE 



Tautomerism. A term introduced by Van Laar, 

 and used to indicate that a compound can react in 

 two different ways. See Isomerism. 



Tetragrammaton. The four letters J H V H (or 

 a variant of them) that in Hebrew texts represent 

 the ineffable name of Jehovah. 



Tissue. Biol. An aggregation of similar cells and 

 fibers that shows a definite structure and is a con- 

 stituent part of an organ. 



Trauma. A wound. 



Tropism. The inherent tendency of an organism to 

 respond in a specific manner to an external stimulus. 

 Thus a heliotropic organism, unless illuminated evenly 

 on all sides, moves either toward or away from the 

 source of light. 



Tyndall {Optical) Effect. In a highly disperse 

 system, the suspended particles are of a size too 

 small to be visible under the microscope when they 

 have diameters of the order of about lO^^ cms. If 

 a powerful converging beam of white light, termed 

 the Tyndall cone, is passed through the disperse 

 system, the suspended particles if viewed at right 

 angles to the direction of the beam, become visible 

 through the light reflections of the individual par- 

 ticles. This is called the Tyndall optical effect. 

 True solutions do not reflect the light. 



Ultra-violet Rays. Light-rays that register be- 

 yond the limit of the visible spectrum. Ultra-violet 

 rays are the actinic, or chemically active, rays. 

 Ultra-violet rays exert a beneficial effect on living 

 beings from the region of the limit of the violet to 

 about 2,900 A; rays of wave-lengths 2,990 A to 

 2,100 A (the middle ultra-violet) have a bactericidal 

 action. 



Urease. A ferment that decomposes urea. 



