Direct chemical action

In presence of an excess of hydrogen, which appears to have no direct chemical action on the aldehyde or its decomposition products, the velocity constant does not decrease for low partial pressures of the aldehyde as it does in the absence of hydrogen. ... 

A toxic is any substance which, by its direct chemical action, either internally or externally, on the human or animal organism, is capable of destroying life or seriously impairing normal body functions. 

Direct Chemical Action of Trichothecenes. Over a quarter of a century ago, Brian et al. (8) described the action of diacetoxyscirpenol on plants when 2.73 X 10" solutions of the metabolite inhibited stem growth and scorched leaves in two pea varities, lettuce, winter tares and other economic crops when applied exogenously. But carrot, beet root, mustard and wheat were not affected at the same application rate signalling genus and species specificity for the, metabolite. However, at rates that ranged from 1.37 x 10"... 

Other hypotheses invoke corrosion in Grignard reactions by direct chemical action l.l()[. No evidence that distinguishes local-cell corrosion from direct chemical action is known to us. However, there is only weak evidence of the discrete Mg(l) intermediates that are required by the hypothesis of direct chemical action (Section 7.3.K). In addition, pitting is a characteristic ol local-cell corrosion, which may also be easier to integrate with the I) model that direct... 

Toxicity. Toxicity is the potential of a substance to cause injury by direct chemical action with the body tissues. Whether the effect is acute or chronic, the only way to avoid such injury is to prevent or greatly minimize contact between toxic chemicals and body tissues. 

If plant hormones are the critical determinants in growth and differentiation great interest attaches to their mechanism of action, to how they control metabolism. Do they exert a direct chemical action on cell structures (such as the cell wall or C3rtoplasmic membranes) altering their ph3rsical properties, do they promote or inhibit enzyme activities, do they intervene in the sequence of chemical events by which the genes control the synthesis of enzymes ... 

Chemically active plastics such as the polyelectrolytes have been used to make artificial muscle materials. This is an unusual type of mechanical power device that creates motion by the lengthening and shortening of fibers made from a chemically active plastic by changing the composition of the surrounding liquid medium, either directly or by the use of electrolytic chemical action. Obviously this form of mechanical power generation is no competitor to thermal energy sources, but it is potentially valuable in detector equipment that would be sensitive to the changing... 

By using direct mechanical action from the artificial muscle, it would be possible to produce reliable sensing and control devices without electrical and electronic equipment. Another interesting application would be to drive prosthetic devices where the action would be similar to the muscle reaction in the body. This unusual type of chemically induced motion should be an interesting one to explore for the solution of unusual problems where conventional approaches do not work. 

Abrasive wear is caused by sharp asperities cutting the plastic fatigue wear is caused by particles of plastic being detached as a result of dynamic stressing on a localised scale adhesive wear is the transfer of plastic to another surface as a result of adhesive forces between the two surfaces. There can also be corrosive wear due to the direct chemical attack on the surface and the term erosive wear is sometimes used for the action of particles in a liquid stream. 

While these steroids can be extracted directly from human tissue, in most instances they can also be synthesized chemically. Direct chemical synthesis methodology has also facilitated the development of synthetic steroid analogues. Many such analogues exhibit therapeutic advantages over the native hormone, e.g. they may be more potent, be absorbed intact from the digestive tract, or exhibit a longer duration of action in the body. The majority of sex steroid hormones now used clinically are chemically synthesized. 

The action of an admixture in relation to attack on reinforcement can be considered either in direct chemical reaction with the steel or, alternatively, a breakdown of the passive layer imparted by concrete which normally prevents corrosion at the cement/steel interface. In this respect, any accelerating water-reducing admixtures containing calcium chloride can be considered hazardous as far as raising susceptibility of steel reinforcement to corrosion is concerned. It is particularly so at calcium chloride contents in the concrete at or above 1.5% by weight of cement as discussed in the section on accelerators. The use of such materials has been controlled by relevant codes of practice where embedded metal is present in the concrete. 

The character of specific chemical actions as described may be illustrated by his discussion of the mortification of metals (from mors death, a term much used by the early alchemists for any process which seemed to deprive metals of their life of spirit. In general it corresponded to any process which we should call direct or indirect oxidation). 

His notion of the cause of chemical action is quite mystical. He supposes a ferment, a formless and unsubstantial something implanted by the divine will in all substances, to be the thing which determines what the action and the products shall be. The various functions of the body, for instance, take place under the initiative of the ferment, and under the guidance or direction of the Areh-aeus, a sort of resident spirit, the concept of the Archaeus being derived, somewhat modified, from the Archaeus of Paracelsus.8... 

The formation of passivating films on lithium in contact with liquid or soluble cathodic reagents is a prerequisite for the construction of a practical cell. The film acts in the same way as a separator, preventing further direct chemical reaction of lithium and the cathodic reagent. However, film formation involving the action of S02, SOCl2, etc. on lithium is considerably more complex and may produce much more severe voltage delay characteristics than in the case of insoluble cathodes described above. 

Wear by roll formation is where there is progressive tearing of a layer of rubber which forms a roll. The result is a characteristic abrasion pattern of ridges and grooves at right angles to the direction of movement. The term erosive wear can be applied to the action of particles conveyed in a liquid stream and there can also be corrosive wear due to direct chemical attack of the surface. 

In black and white photography, the developed visible image consists of particles of silver produced by the chemical action. By suitable processes, dye images can be formed that are related directly to the amount of developed silver... 

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