def _send_rtm_message(self, channel_id, text):
        """Send a Slack message to a channel over RTM.

        :param channel_id: a slack channel id.
        :param text: a slack message. Serverside formatting is done
          in a similar way to normal user message; see
          `Slack's docs <https://api.slack.com/docs/formatting>`__.
        """

        message = {
            'id': self._current_message_id,
            'type': 'message',
            'channel': channel_id,
            'text': text,
        }
        self.ws.send(json.dumps(message))
        self._current_message_id += 1