film referred to as 34 epi cuticle 34 4 are as a whole furnished with the water repelling property and are hardly wetted however on the back of the scales 1 are a protein layer referred to as 34 endo cuticle 34 3 that contains a great number of polar groups and is open at the inter scale gaps thus allowing easy passage of the water under the normal condition the animal fiber contains about 15 water as the inherent moisture at the open portions of the scales when an acid or a neutral salt is about to be dissolved in this inherent moisture such as when hydrochloric acid is used in combination with a saturated sodium chloride solution for example the hydrogen ion of hydrochloric acid exhibits the diffusion velocity of 4 to 5 times that of the sodium ion of sodium chloride even at normal temperature and hydrochloric acid is dissolved more rapidly than sodium chloride so that the acid is contained only in the portions of the endo cuticles 3 with the other portions being un corroded by the acid in this case a surfactant that would possibly hinder the saturated solution of the neutral salt used should never be employed when the animal fiber is exposed in the current of hydrogen chloride gas or the like the.