159     Two substances, opposite to each other, united into neutral Natrum and middle salts by chemical affinity in unchangeable proportions, as well as sulfuretted metals found in the earth and those produced by technical art in constant combining proportions of sulphur and alkaline salts and earths, for instance (natrum sulph. and calcarea sulph.) as well as those ethers produced by distillation of alcohol and acids may together with phosphorus be considered as simple medicinal substances by the homœopathic physician and used for patients. On the other hand, those extracts obtained by means of acids of the so-called alkaloids of plants, are exposed to great variety in their preparation (for instance, chinin, strychnine, morphine), and can, therefore, not be accepted by the homœopathic physician as simple medicines, always the same, especially as he possesses, in the plants themselves, in their natural state (Peruvian bark, nux vomica, opium) every quality necessary for healing. Moreover, the alkaloids are not the only constituents of the plants.