this is now considered erroneous since short-range order in water only persists for about 1 picosecond. No evidence of stable clusters of water molecules was found when homeopathic preparations were studied using nuclear magnetic resonance, and many other physical experiments in homeopathy have been found to be of low methodological quality, which precludes any meaningful conclusion. Existence of a pharmacological effect in the absence of any true active ingredient is inconsistent with the law of mass action and the observed dose-response relationships characteristic of therapeutic drugs (whereas placebo effects are non-specific and unrelated to pharmacological activity). Homeopaths contend that their methods produce a therapeutically active preparation, selectively including only the intended substance, though critics note that any water will have been in contact with millions of different substances throughout its history, and homeopaths have not been able to account for a reason why only the selected homeopathic substance would be a special case in their process. For comparison, ISO 3696:1987 defines a standard for water used in laboratory analysis; this allows for a contaminant level of ten parts per billion, 4C in homeopathic notation. This water may not be kept in glass as contaminants will leach out into the water. Practitioners of homeopathy hold that higher dilutions―described as being of higher potency―produce stronger medicinal effects. This idea is also inconsistent with observed dose-response relationships, where effects are dependent on the concentration of the active ingredient in the body. This dose-response relationship has been confirmed in myriad experiments on organisms as diverse as nematodes, rats, and humans. Some homeopaths contend that the phenomenon of hormesis may support the idea of dilution increasing potency, but the dose-response relationship outside the zone of hormesis declines with dilution as normal, and nonlinear pharmacological effects do not provide any credible support for homeopathy. Physicist Robert L. Park, former executive director of the American Physical Society, is quoted as saying:"since the least amount of a substance in a solution is one molecule, a 30C solution would have to have at least one molecule of the original substance dissolved in a minimum of 1,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 [or 1060] molecules of water. This would require a container more than 30,000,000,000 times the size of the Earth." Park is also quoted as saying that, "to expect to get even one molecule of the 'medicinal' substance allegedly present in 30X pills, it would be necessary to take some two billion of them,