Homogeneous groups

The alkali metals form a homogeneous group of extremely reactive elements which illustrate well the similarities and trends to be expected from the periodic classification, as discussed in Chapter 2. Their physical and chemical properties are readily interpreted in terms of their simple electronic configuration, ns, and for this reason they have been extensively studied by the full range of experimental and theoretical techniques. Compounds of sodium and potassium have been known from ancient times and both elements are essential for animal life. They are also major items of trade, commerce and chemical industry. Lithium was first recognized as a separate element at the beginning of the nineteenth eentury but did not assume major industrial importance until about 40 y ago. Rubidium and caesium are of considerable academic interest but so far have few industrial applications. Francium, the elusive element 87, has only fleeting existence in nature due to its very short radioactive half-life, and this delayed its discovery until 1939. 

Scientific data points usually require curves, several lines, or both to divide them into homogeneous groups. 

Waxes do not form a homogeneous group from both chemical and biological points of view. They are made of a great variety of molecular compounds and they originate from... 

An important theme to this review is that people respond to food crises differently. We can still, however, achieve a reasonable degree of predictability by segmenting them into somewhat homogeneous groups based—not on demographics—but on a person s risk perceptions and risk attitude. For every crisis there will be accountable, concerned, conservative, and alarmist segments of consumers. Knowing their relative size will enable us to better predict the effectiveness of different interventions. 

This combination of an apparently infinite range of antibody specificity associated with what appeared to be a nearly homogenous group of proteins astonished me and still does. 

In addition to the toxicokinetic and toxicodynamic differences mentioned above, other aspects of differences between experimental animals and humans include different types of organs and tissues, differences in digestion, and differences in the stmcture of the upper respiratory tract. Furthermore, animal studies are performed in homogenous groups of animals, but the results have to be apphed for the protection of all individuals in a heterogeneous population of humans. In consequence of this, interspecies variation must also be expected. 

When tumors become measurable (about 100 mm ), randomize mice in two homogenous groups one with and another without tetracycline, which is provided by dissolving it directly in drinking water (2 mg/ml). Replace tetracycline-supplemented drinking water every 2 days (see Note 23). 

In one sense the signal is out of our control, it will depend entirely on what the true treatment difference is. Similarly there is little we can do about the patient-to-patient variability, although we can reduce this by having, for example, precise measures of outcome or a more homogeneous group of patients. The sample size however is very much under our control and common sense tells us that increasing this will provide a more reliable comparison and make it easier for us to detect treatment differences when they exist. 

However, the hot evolved stars do not form a homogeneous group. The most prominent division is into Central Stars of Planetary Nebulae differences exist in spectral type and chemical composition (as deduced from medium and high resolution spectra). Consequently, a unique progenitor for all kinds of hot evolved stars appears to be very unlikely and accurate stellar parameters are needed to compare these stars to predictions of stellar evolution theory. 

Alkaloids are extremely difficult to define because they do not represent a homogeneous group of compounds from either the chemical, biochemical or physiological viewpoint. All do occur in plants, but some are found in animals, and practically all have been reproduced in the laboratory by chemical synthesis. Most possess basic properties due to the presence of an amino nitrogen, and many, specially thoses pertinent to pharmacy and medicine, possess marked physiological activity. 

It seems clear that for reactions of carbocations with nucleophiles or bases in which the structure of the carbocation is varied, we can expect compensating changes in intrinsic barrier and thermodynamic driving force to lead to relationships between rate and equilibrium constants which have the form of extended linear plots of log k against log K. However, this will be strictly true only for structurally homogeneous groups of cations. There is ample evidence that for wider structural variations, for example, between benzyl, benzhydryl, and trityl cations, there are variations in intrinsic barrier particularly reflecting steric effects which lead to dispersion between families of cations. 

Urophylleae, Psychotrieae, Naucleae, and Cinchoneae. Only some genera of each of these tribes are concerned and constitute thus a rather homogeneous group Adina, Anthocephalus, Nauclea, Uncaria, Pauridiantha, and Palicourea. 

---