Whenever a subsequent injury or aggravation of a previous injury or pre-existing condition occurs to an employee, the risk of the employer for whom such person was working at the time of such subsequent injury or aggravation shall be charged only with the amount of the awards     resulting from such subsequent injury or aggravation. Whenever such subsequent injury or aggravation results in further disability or an aggravation of a pre-existing injury or condition, the compensation which is in excess of the amount to which the injured employee would have been entitled solely by reason of the subsequent injury or aggravation shall be charged to the subsequent injury fund and not to the classification or the risk to which the subsequent injury or aggravation is charged.