§ 113. Leaves of absence to professional members of the department. The department, subject to such regulations as the commissioner may promulgate, may grant any professional member of the staff of the department an extended leave of absence for advanced study, research, professional writing or other experience of recognized professional value, for not to exceed one year in seven at the rate of not to exceed one-half the regular salary paid, or for not to exceed six months with salary at full rate for such leave. The balance of salaries appropriated for such persons, or so much thereof as may be necessary, may be applied during the period of such absence for the purpose of the salaries of substitutes to take the place of the persons thus on leave. Not more than five such leaves of absence shall be granted during any one fiscal year.